Magnetic tape having characterized psd ratio, magnetic tape cartridge, and magnetic recording and reproducing apparatus

ABSTRACT

The magnetic tape includes a non-magnetic support, a magnetic layer that includes ferromagnetic powder having an average particle volume of 2,500 nm 3  or less on one surface side of the non-magnetic support, and a back coating layer that includes non-magnetic powder on the other surface side of the non-magnetic support, in which the ferromagnetic powder is ferromagnetic powder selected from the group consisting of hexagonal ferrite powder and ε-iron oxide powder, and a ratio (PSD 5μm-PSDmag /PSD 10μm-PSDbc ) of the magnetic layer and the back coating layer is in a range of 0.0050 to 0.20. A magnetic tape cartridge and a magnetic recording and reproducing apparatus include the magnetic tape.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 16/802,797filed Feb. 27, 2020, which claims priority under 35 U.S.C 119 toJapanese Patent Application No. 2019-036711 filed on Feb. 28, 2019. Theabove applications are hereby expressly incorporated by reference, intheir entirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic tape, a magnetic tapecartridge, and a magnetic recording and reproducing apparatus.

2. Description of the Related Art

There are a tape-shaped magnetic recording medium and a disk-shapedmagnetic recording medium, and a tape-shaped magnetic recording medium,that is, a magnetic tape is mainly used for data storage applicationssuch as data backup and archive. For the magnetic tape, a back coatinglayer is provided on a surface side of a non-magnetic support oppositeto a surface side provided with a magnetic layer (for example, seeJP2004-030870A).

SUMMARY OF THE INVENTION

A magnetic layer of a magnetic tape includes ferromagnetic powder. Inrecent years, from a viewpoint of high density recording adequacy or thelike, hexagonal ferrite powder and ε-iron oxide powder are attractingattention as ferromagnetic powder. Furthermore, a high recording densityis always desired for the magnetic tape. As means for increasing therecording density of the magnetic tape, ferromagnetic powder having asmall particle volume is used as the ferromagnetic powder included inthe magnetic layer.

However, in a view of the above, the present inventor has studied amagnetic tape that includes a magnetic layer and a back coating layer,the magnetic layer including ferromagnetic powder having a smallparticle volume selected from the group consisting of hexagonal ferritepowder and ε-iron oxide powder, and as a result, it has become clearthat improvement in recording quality and/or reproducing quality whenthe magnetic tape is repeatedly run (hereinafter, referred to as“recording and reproducing quality during repeated running”) is desired.Recording and reproducing quality during repeated running means that,for example, in a case where data (information) is recorded on amagnetic tape after the magnetic tape is repeatedly run, recording andreproducing quality during repeated running is excellent in a case wherethe data can be continuously recorded stably in a longitudinal directionof the magnetic tape.

An object of an aspect of the present invention is to provide a magnetictape that includes a magnetic layer and a back coating layer, themagnetic layer including ferromagnetic powder having a small particlevolume selected from the group consisting of hexagonal ferrite powderand ε-iron oxide powder, and is excellent in recording and reproducingquality during repeated running.

An aspect of the present invention relates to a magnetic tapecomprising: a non-magnetic support; a magnetic layer that includesferromagnetic powder having an average particle volume of 2,500 nm³ orless on one surface side of the non-magnetic support; and a back coatinglayer that includes non-magnetic powder on the other surface side of thenon-magnetic support, in which the ferromagnetic powder is ferromagneticpowder selected from the group consisting of hexagonal ferrite powderand ε-iron oxide powder, and a ratio (PSD_(5μm-PSDmag)/PSD_(10μm-PSDbc))of a PSD_(5μm-PSDmag) at a 5 μm pitch on a surface of the magnetic layerand a PSD_(10μm-PSDbc) at a 10 μm pitch on a surface of the back coatinglayer is in a range of 0.0050 to 0.20. A “PSD” refers to power spectrumdensity.

In an aspect, a ratio (PSD_(3μm-PSDbc)/PSD_(10μm-PSDbc)) of aPSD_(3μm-PSDbc) at a 3 μm pitch on a surface of the back coating layerand a PSD_(10μm-PSDbc at a) 10 μm pitch on a surface of the back coatinglayer may be in a range of 0.050 to 0.75.

In an aspect, a product (Rku_(mag)×Rku_(bc)) of a kurtosis Rku_(mag) ofa surface of the magnetic layer and a kurtosis Rku_(bc) of a surface ofthe back coating layer may be in a range of 7.0 to 20.0.

In an aspect, the kurtosis Rku_(mag) of the surface of the magneticlayer and the kurtosis Rku_(bc) of the surface of the back coating layermay have a relationship of Rku_(mag)<Rku_(bc).

In an aspect, at least one of a skewness Rsk_(mag) of a surface of themagnetic layer or a skewness Rsk_(bc) of a surface of the back coatinglayer may be 0 or more.

In an aspect, the skewness Rsk_(bc) of the surface of the back coatinglayer may be 0 or more.

In an aspect, the hexagonal ferrite powder may be hexagonal strontiumferrite powder.

Another aspect of the present invention relates to a magnetic tapecartridge comprising: the magnetic tape described above.

Another aspect of the present invention relates to a magnetic recordingand reproducing apparatus comprising: the magnetic tape described above;and a magnetic head.

According to an aspect of the present invention, it is possible toprovide a magnetic tape that includes a magnetic layer and a backcoating layer, the magnetic layer including ferromagnetic powder havinga small particle volume selected from the group consisting of hexagonalferrite powder and ε-iron oxide powder, and is excellent in recordingand reproducing quality during repeated running According to an aspectof the present invention, it is possible to provide a magnetic tapecartridge and a magnetic recording and reproducing apparatus whichinclude such a magnetic tape.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Magnetic Tape

An aspect of the present invention relates to a magnetic tapecomprising: a non-magnetic support; a magnetic layer that includesferromagnetic powder having an average particle volume of 2,500 nm³ orless on one surface side of the non-magnetic support; and a back coatinglayer that includes non-magnetic powder on the other surface side of thenon-magnetic support, in which the ferromagnetic powder is ferromagneticpowder selected from the group consisting of hexagonal ferrite powderand ε-iron oxide powder, and a ratio (PSD_(5μm-PSDmag)/PSD_(10μm-PSDbc))of a PSD_(5μm-PSDmag) at a 5 μm pitch on a surface of the magnetic layerand a PSD_(10μm-PSDbc) at a 10 μm pitch on a surface of the back coatinglayer is in a range of 0.0050 to 0.20.

Hereinafter, the magnetic tape will be described more specifically.

Surface Shape of Magnetic Layer and Back Coating Layer

Ratio (PSD_(5μm-PSDmag)/PSD_(10μm-PSDbc))

For example, a PSD at a 10 μm pitch is obtained by the following method.

A PSD is measured using a non-contact optical interference type surfaceroughness machine. For example, a non-contact optical interference typesurface roughness machine manufactured by Bruker Japan K.K., WYKOCorporation, or Zygo Corporation can be used. At the time ofmeasurement, it is preferable to use an objective lens having a highmagnification (for example, about 50 times). A magnification of anobjective lens and an intermediate lens is set so that a sampling lengthis in a range of 50 nm to 300 nm, and profile data of a measurementtarget surface is measured.

A PSD in a longitudinal direction of the magnetic tape is calculated.The profile data in the longitudinal direction is Fourier transformed bya mounting function of the non-contact optical interference type surfaceroughness machine, and the averaged data is calculated as a PSD.

From this PSD, a PSD value at each wavelength is calculated to obtain aPSD value corresponding to a 10 μm pitch. The PSD value thus obtained isset as a PSD at a 10 μm pitch. The same applies to PSDs at otherpitches. For specific aspects of a method of measuring the PSD, thefollowing examples can be referred to.

The PSD obtained by the above method is a value that can serve as anindicator of an existence state of a waviness component on a surface ofa measurement target layer. While the present inventor has repeatedlystudied, it has been found that in a magnetic tape that includes anon-magnetic support, a magnetic layer including ferromagnetic powderhaving an average particle volume of 2,500 nm³ or less on one surfaceside of the non-magnetic support, and a back coating layer includingnon-magnetic powder on the other surface side of the non-magneticsupport, an existence state of waviness components on surfaces of themagnetic layer and the back coating layer is controlled so that a ratio(PSD_(5μm-PSDmag)/PSD_(10μm-PSDbc)) is in a range of 0.0050 to 0.20, andthus it becomes possible to improve recording and reproducing qualityduring repeated running. This point will be further described below.

The magnetic tape is usually wound around a reel of a magnetic tapecartridge and accommodated in the magnetic tape cartridge. The magnetictape cartridge is mounted on a magnetic recording and reproducingapparatus (called a drive), and the magnetic tape is fed out from thereel of the magnetic tape cartridge in the drive or wound on the reel ofthe magnetic tape cartridge, whereby the magnetic tape can be run in thedrive. It is considered that a winding deviation between a magneticlayer surface and a back coating layer surface during winding on thereel causes damage on the magnetic layer surface. The damage on themagnetic layer surface can cause a change in spacing between themagnetic layer surface and a magnetic head in a case where the magnetictape and the magnetic head come into contact with each other to be slidon each other for recording data and/or reproducing the recorded data onthe magnetic tape. In this regard, it is supposed that the smaller aparticle volume of ferromagnetic powder included in a magnetic layer is,the smaller the number of molecules of a binding agent with respect toferromagnetic particles constituting the ferromagnetic powder is, andtherefore the magnetic layer is likely to be brittle and easily damaged.In addition, it is supposed that a magnetic layer including ε-iron oxidepowder generally tends to have a high anisotropy magnetic field Hk, andtherefore it is easily affected by spacing change during data recording.A magnetic layer including hexagonal strontium ferrite powder inhexagonal ferrite powder generally tends to have a high anisotropymagnetic field Hk, and therefore the same applies thereto.

With respect to this, it is considered that an existence state ofwaviness components on surfaces of the magnetic layer and the backcoating layer is controlled so that a ratio(PSD_(5μm-PSDmag)/PSD_(10μm-PSDbc)) is in a range of 0.0050 to 0.20, andthus a so-called wedge effect can be exerted between the magnetic layersurface and the back coating layer surface during winding onto a reel.It is supposed that this leads to suppression of a winding deviationbetween the magnetic layer surface and the back coating layer surfaceduring winding onto the reel, and it becomes possible to improverecording and reproducing quality during repeated running. However, theabove is supposition and does not limit the present invention.Furthermore, the present invention is not limited to other suppositionsdescribed in this specification.

In the above magnetic tape, the ratio(PSD_(5μm-PSDmag)/PSD_(10μm-PSDbc)) is 0.0050 or more, and from aviewpoint of further improving the recording and reproducing quality, itis preferably 0.010 or more, more preferably 0.020 or more, and stillmore preferably 0.030 or more. In addition, in the above magnetic tape,the ratio (PSD_(5μm-PSDmag)/PSD_(10μm-PSDbc)) is 0.20 or less, and froma viewpoint of further improving the recording and reproducing quality,it is preferably 0.15 or less, more preferably 0.10 or less, still morepreferably 0.080 or less, and still more preferably 0.060 or less.

In the above magnetic tape, the ratio(PSD_(5μm-PSDmag)/PSD_(10μm-PSDbc)) may be in the above range, aPSD_(5μm-PSDmag) at a 5 μm pitch on a surface of the magnetic layer anda PSD_(10μm-PSDbc) at a 10 μm pitch on a surface of the back coatinglayer are not limited. The PSD_(5μm-PSDmag) at a 5 μm pitch on thesurface of the magnetic layer can be, for example, 3.00 E+02 nm³ ormore, preferably 6.00 E+02 nm³ or more, and more preferably 1.00 E+03nm³ or more. Moreover, the PSD_(5μm-PSDmag) at a 5 μm pitch on thesurface of the magnetic layer can be, for example, 1.50 E+04 nm³ orless, preferably 9.00 E+03 nm³ or less, and more preferably 3.00 E+03nm³ or less. The PSD_(10μm-PSDbc) at a 10 μm pitch on the surface of theback coating layer can be, for example, 1.00 E+04 nm³ or more,preferably 3.00 E+04 nm³ or more, and more preferably 5.00 E+04 nm³ ormore. Moreover, the PSD_(10μm-PSDbc) at a 10 μm pitch on the surface ofthe back coating layer can be, for example, 5.00 E+05 nm³ or less,preferably 3.00 E+05 nm³ or less, and more preferably 1.00 E+05 nm³ orless. In an aspect, a PSD_(10μm-PSDbc) can be in a range of 2.00 E+04 to8.00 E+04 nm³. In an aspect, a PSD_(10μm-PSDmag) at a 10 μm pitch on thesurface of the magnetic layer can be in a range of 8.00 E+02 to 1.00E+04 nm³. As is well known, “E” is an exponential notation, for example,“E+03” indicates “×10³” (cube of 10).

Ratio (PSD_(3μm-PSDbc)/PSD_(10μm-PSDbc))

In the above magnetic tape, from a viewpoint of further improvingrecording and reproducing quality during repeated running, a ratio(PSD_(3μm-PSDbc)/PSD_(10μm-PSDbc)) of a PSD_(3μm-PSDbc) at a 3 μm pitchon a surface of the back coating layer and a PSD_(10μm-PSDbc) at a 10 μmpitch on a surface of the back coating layer is preferably in a range of0.050 to 0.75. It is considered that the ratio(PSD_(3μm-PSDbc)/PSD_(10μm-PSDbc)) of 0.050 or more contributes tofurther suppression of a winding deviation between the magnetic layersurface and the back coating layer surface during winding onto the reel.Further, the smaller ratio (PSD_(3μm-PSDbc)/PSD_(10μm-PSDbc)) isconsidered to mean that the change in roughness in a minute region onthe back coating layer surface becomes smaller. It is supposed that thesmaller the change in roughness is, the magnetic layer surface is lesslikely damaged due to the roughness of the back coating layer surface.With respect to this, it is considered that the ratio(PSD_(3μm-PSDbc)/PSD_(10μm-PSDbc)) of 0.75 or less leads to reduction indamage on the magnetic layer surface due to the roughness of the backcoating layer. From a viewpoint of further improving the recording andreproducing quality during repeated running, the ratio(PSD_(3μm-PSDbc)/PSD_(10μm-PSDbc)) is more preferably 0.070 or more,still more preferably 0.10 or more, still more preferably 0.20 or more,and still more preferably 0.30 or more. From the same viewpoint, theratio (PSD_(3μm-PSDbc)/PSD_(10μm-PSDbc)) is more preferably 0.70 orless, and still more preferably 0.60 or less. The PSD_(3μm-PSDbc) at a 3μm pitch on a surface of the back coating layer can be, for example, ina range of 1.00 E+03 to 1.00 E+05 nm³.

Product (Rku_(mag)×Rku_(bc))

In the above magnetic tape, from a viewpoint of further improving therecording and reproducing quality during repeated running, a product(Rku_(mag)×Rku_(bc)) of a kurtosis Rku_(mag) of a surface of themagnetic layer and a kurtosis Rku_(bc) of a surface of the back coatinglayer is preferably in a range of 7.0 to 20.0. A kurtosis Rku is a valueobtained according to JIS B 0601:2013 from profile data of a surfaceroughness in a longitudinal direction of the magnetic tape obtained fora region having an area 167 μm×125 μm on a surface of a measurementtarget layer by using a non-contact optical interference type surfaceroughness machine. For specific aspects of a method of measuring theRku_(mag) and Rku_(bc), the following examples can be referred to. Thekurtosis Rku represents a sharpness of a height distribution of thesurface, “Rku=3” represents that the height distribution is a normaldistribution, “Rku>3” represents that the surface has many sharpirregularities, and “Rku<3” represents that the surface is flat with fewsharp irregularities. It is considered that the more sharpirregularities on the magnetic layer surface and/or the back coatinglayer surface are, the more a so-called wedge effect can be exerted, anda winding deviation between the magnetic layer surface and the backcoating layer surface during winding onto the reel can be furthersuppressed. From this viewpoint, the product (Rku_(mag)×Rku_(bc)) ispreferably 7.0 or more, more preferably 8.0 or more, and still morepreferably 9.0 or more. From a viewpoint of suppressing damage to beeasily generated on the magnetic layer surface and/or the back coatinglayer surface due to a protrusion, the product (Rku_(mag)×Rku_(bc)) ispreferably 20.0 or less, more preferably 18.0 or less, still morepreferably 16.0 or less, still more preferably 14.0 or less, and stillmore preferably 12.0 or less. From a viewpoint of further suppressingdamage to be easily generated on the magnetic layer surface, it ispreferable that a kurtosis Rku_(mag) of a surface of the magnetic layeris smaller than a kurtosis Rku_(bc) of a surface of the back coatinglayer, that is, a relationship of Rku_(mag)<Rku_(bc) is satisfied. Thekurtosis Rku_(mag) of a surface of the magnetic layer is preferably in arange of 2.00 to 6.00, and more preferably in a range of 2.50 to 4.00.The kurtosis Rku_(bc) of a surface of the back coating layer ispreferably in a range of 2.00 to 6.00, and more preferably in a range of2.50 to 5.00.

Rsk_(mag) and Rsk_(bc)

In the above magnetic tape, from a viewpoint of further improving therecording and reproducing quality during repeated running, at least oneof a skewness Rsk_(mag) of a surface of the magnetic layer or a skewnessRsk_(bc) of a surface of the back coating layer is preferably 0 or more.A skewness Rsk is a value obtained according to JIS B 0601:2013 fromprofile data of a surface roughness in a longitudinal direction of themagnetic tape obtained for a region having an area 167 μm×125 μm on asurface of a measurement target layer by using a non-contact opticalinterference type surface roughness machine. For specific aspects of amethod of measuring the Rsk_(mag) and Rsk_(bc), the following examplescan be referred to. The skewness Rsk represents a symmetry of a heightdistribution of the surface, “Rsk=0” represents that the heightdistribution (vertical axis is height) is vertically symmetric, “Rsk>0”(that is, Rsk is positive value) represents that the surface has manyprotrusions, and “Rsk<0” (that is, Rsk is negative value) representsthat the surface has many recesses. From a viewpoint of further exertingthe so-called wedge effect, at least one of Rsk_(mag) or Rsk_(bc) ispreferably 0 or more, and more preferably more than 0. Rsk_(mag) can be,for example, in a range of −0.75 to 0.75 (that is, +0.75), and Rsk_(bc)can be, for example, in a range of −0.50 to 1.00 (that is, +1.00). In anaspect, Rsk_(mag) and Rsk_(bc) can be 0 or more or more than 0. Further,from a viewpoint of high density recording, since it is preferable toreduce the protrusion on the magnetic layer surface, Rsk_(mag) ispreferably less than 0. Rsk_(bc) can be 0 or more, more than 0, or lessthan 0, preferably 0 or more or more than 0, and more preferably morethan 0.

The method for controlling the surface shape of the magnetic layer andthe back coating layer described above will be described later.

Magnetic Layer

Ferromagnetic Powder

Average Particle Volume

The above magnetic tape includes, in a magnetic layer, ferromagneticpowder having an average particle volume of 2,500 nm³ or less which isselected from the group consisting of hexagonal ferrite powder andε-iron oxide powder. An average particle volume is a value obtained as asphere equivalent volume, from an average particle size obtained by amethod which will be described later. Ferromagnetic powder having anaverage particle volume of 2,500 nm³ or less included in a magneticlayer is preferable from a viewpoint of improving recording density.From this viewpoint, the average particle volume is preferably 2,300 nm³or less, more preferably 2,000 nm³ or less, and still more preferably1,500 nm³ or less. On the other hand, from a viewpoint of magnetizationstability, the average particle volume is preferably 500 nm³ or more,more preferably 600 nm³ or more, still more preferably 650 nm³ or more,and still more preferably 700 nm³ or more. A magnetic layer of the abovemagnetic tape can include one or more ferromagnetic powders selectedfrom the group consisting of hexagonal ferrite powder and ε-iron oxidepowder.

Hexagonal Ferrite Powder

In an aspect, the above magnetic tape may include hexagonal ferritepowder in the magnetic layer. For details of the hexagonal ferritepowder, for example, descriptions disclosed in paragraphs 0012 to 0030of JP2011-225417A, paragraphs 0134 to 0136 of JP2011-216149A, paragraphs0013 to 0030 of JP2012-204726A, and paragraphs 0029 to 0084 ofJP2015-127985A can be referred to.

In the present invention and this specification, “hexagonal ferritepowder” refers to ferromagnetic powder in which a hexagonal ferrite typecrystal structure is detected as a main phase by X-ray diffractionanalysis. The main phase refers to a structure to which the highestintensity diffraction peak in an X-ray diffraction spectrum obtained byX-ray diffraction analysis is attributed. For example, in a case wherethe highest intensity diffraction peak is attributed to a hexagonalferrite type crystal structure in an X-ray diffraction spectrum obtainedby X-ray diffraction analysis, it is determined that the hexagonalferrite type crystal structure is detected as the main phase. In a casewhere only a single structure is detected by X-ray diffraction analysis,this detected structure is taken as the main phase. The hexagonalferrite type crystal structure includes at least an iron atom, adivalent metal atom and an oxygen atom, as a constituent atom. Thedivalent metal atom is a metal atom that can be a divalent cation as anion, and examples thereof may include an alkaline earth metal atom suchas a strontium atom, a barium atom, and a calcium atom, a lead atom, andthe like. In the present invention and this specification, hexagonalstrontium ferrite powder means that the main divalent metal atomincluded in the powder is a strontium atom, and the hexagonal bariumferrite powder means that the main divalent metal atom included in thispowder is a barium atom. The main divalent metal atom refers to adivalent metal atom that accounts for the most on an at % basis amongdivalent metal atoms included in the powder. Here, a rare earth atom isnot included in the above divalent metal atom. The “rare earth atom” inthe present invention and this specification is selected from the groupconsisting of a scandium atom (Sc), an yttrium atom (Y), and alanthanoid atom. The Lanthanoid atom is selected from the groupconsisting of a lanthanum atom (La), a cerium atom (Ce), a praseodymiumatom (Pr), a neodymium atom (Nd), a promethium atom (Pm), a samariumatom (Sm), a europium atom (Eu), a gadolinium atom (Gd), a terbium atom(Tb), a dysprosium atom (Dy), a holmium atom (Ho), an erbium atom (Er),a thulium atom (Tm), an ytterbium atom (Yb), and a lutetium atom (Lu).

Hereinafter, hexagonal strontium ferrite powder which is an aspect ofthe hexagonal ferrite powder will be described in more detail.

An index for reducing thermal fluctuation, in other words, improvingthermal stability may include an anisotropy constant Ku. The hexagonalstrontium ferrite powder may preferably have Ku of 1.8×10⁵ J/m³ or more,and more preferably have a Ku of 2.0×10⁵ J/m³ or more. Ku of thehexagonal strontium ferrite powder may be, for example, 2.5×10⁵ J/m³ orless. Here, it means that the higher Ku is, the higher thermal stabilityis, this is preferable, and thus, a value thereof is not limited to thevalues exemplified above. For a unit of the anisotropy constant Ku, 1erg/cc=1.0×10⁻¹ J/m³.

The hexagonal strontium ferrite powder may or may not include a rareearth atom. In a case where the hexagonal strontium ferrite powderincludes a rare earth atom, it is preferable to include a rare earthatom at a content (bulk content) of 0.5 to 5.0 at % with respect to 100at % of an iron atom. In an aspect, the hexagonal strontium ferritepowder including a rare earth atom may have a rare earth atom surfacelayer portion uneven distribution property. In the present invention andthis specification, the “rare earth atom surface layer portion unevendistribution property” means that a rare earth atom content with respectto 100 at % of an iron atom in a solution obtained by partiallydissolving hexagonal strontium ferrite powder with an acid (hereinafter,referred to as a “rare earth atom surface layer portion content” orsimply a “surface layer portion content” for a rare earth atom) and arare earth atom content with respect to 100 at % of an iron atom in asolution obtained by totally dissolving hexagonal strontium ferritepowder with an acid (hereinafter, referred to as a “rare earth atom bulkcontent” or simply a “bulk content” for a rare earth atom) satisfy aratio of a rare earth atom surface layer portion content/a rare earthatom bulk content>1.0. A rare earth atom content in hexagonal strontiumferrite powder which will be described later is the same meaning as therare earth atom bulk content. On the other hand, partial dissolutionusing an acid dissolves a surface layer portion of a particleconfiguring hexagonal strontium ferrite powder, and thus, a rare earthatom content in a solution obtained by partial dissolution is a rareearth atom content in a surface layer portion of a particle configuringhexagonal strontium ferrite powder. A rare earth atom surface layerportion content satisfying a ratio of “rare earth atom surface layerportion content/rare earth atom bulk content>1.0” means that in aparticle of hexagonal strontium ferrite powder, rare earth atoms areunevenly distributed in a surface layer portion (that is, more than aninside). The surface layer portion in the present invention and thisspecification means a partial region from a surface of a particleconfiguring hexagonal strontium ferrite powder toward an inside.

In a case where hexagonal strontium ferrite powder includes a rare earthatom, a rare earth atom content (bulk content) is preferably in a rangeof 0.5 to 5.0 at % with respect to 100 at % of an iron atom. It isconsidered that a bulk content in the above range of the included rareearth atom and uneven distribution of the rare earth atoms in a surfacelayer portion of a particle configuring hexagonal strontium ferritepowder contribute to suppression of a decrease in a reproducing outputin repeated reproduction. It is supposed that this is because hexagonalstrontium ferrite powder includes a rare earth atom with a bulk contentin the above range, and rare earth atoms are unevenly distributed in asurface layer portion of a particle configuring hexagonal strontiumferrite powder, and thus it is possible to increase an anisotropyconstant Ku. The higher a value of an anisotropy constant Ku is, themore a phenomenon called so-called thermal fluctuation can be suppressed(in other words, thermal stability can be improved). By suppressingoccurrence of thermal fluctuation, it is possible to suppress a decreasein reproducing output during repeated reproduction. It is supposed thatuneven distribution of rare earth atoms in a particulate surface layerportion of hexagonal strontium ferrite powder contributes tostabilization of spins of iron (Fe) sites in a crystal lattice of asurface layer portion, and thus, an anisotropy constant Ku may beincreased.

Moreover, it is supposed that the use of hexagonal strontium ferritepowder having a rare earth atom surface layer portion unevendistribution property as a ferromagnetic powder in the magnetic layeralso contributes to inhibition of a magnetic layer surface from beingscraped by being slid with respect to the magnetic head. That is, it issupposed that hexagonal strontium ferrite powder having rare earth atomsurface layer portion uneven distribution property can also contributeto an improvement of running durability of the magnetic tape. It issupposed that this may be because uneven distribution of rare earthatoms on a surface of a particle configuring hexagonal strontium ferritepowder contributes to an improvement of interaction between the particlesurface and an organic substance (for example, a binding agent and/or anadditive) included in the magnetic layer, and, as a result, a strengthof the magnetic layer is improved.

From a viewpoint of further suppressing a decrease in reproducing outputduring repeated reproduction and/or a viewpoint of further improving therunning durability, the rare earth atom content (bulk content) is morepreferably in a range of 0.5 to 4.5 at %, still more preferably in arange of 1.0 to 4.5 at %, and still more preferably in a range of 1.5 to4.5 at %.

The bulk content is a content obtained by totally dissolving hexagonalstrontium ferrite powder. In the present invention and thisspecification, unless otherwise noted, the content of an atom means abulk content obtained by totally dissolving hexagonal strontium ferritepowder. The hexagonal strontium ferrite powder including a rare earthatom may include only one kind of rare earth atom as the rare earthatom, or may include two or more kinds of rare earth atoms. The bulkcontent in the case of including two or more types of rare earth atomsis obtained for the total of two or more types of rare earth atoms. Thisalso applies to other components in the present invention and thisspecification. That is, unless otherwise noted, a certain component maybe used alone or in combination of two or more. A content amount orcontent in a case where two or more components are used refers to thatfor the total of two or more components.

In a case where the hexagonal strontium ferrite powder includes a rareearth atom, the included rare earth atom may be any one or more of rareearth atoms. As a rare earth atom that is preferable from a viewpoint offurther suppressing a decrease in reproducing output in repeatedreproduction, there are a neodymium atom, a samarium atom, a yttriumatom, and a dysprosium atom, here, the neodymium atom, the samariumatom, and the yttrium atom are more preferable, and a neodymium atom isstill more preferable.

In the hexagonal strontium ferrite powder having a rare earth atomsurface layer portion uneven distribution property, the rare earth atomsmay be unevenly distributed in the surface layer portion of a particleconfiguring the hexagonal strontium ferrite powder, and the degree ofuneven distribution is not limited. For example, for a hexagonalstrontium ferrite powder having a rare earth atom surface layer portionuneven distribution property, a ratio between a surface layer portioncontent of a rare earth atom obtained by partial dissolution underdissolution conditions which will be described later and a bulk contentof a rare earth atom obtained by total dissolution under dissolutionconditions which will be described later, that is, “surface layerportion content/bulk content” exceeds 1.0 and may be 1.5 or more. A“surface layer portion content/bulk content” larger than 1.0 means thatin a particle configuring the hexagonal strontium ferrite powder, rareearth atoms are unevenly distributed in the surface layer portion (thatis, more than in the inside). Further, a ratio between a surface layerportion content of a rare earth atom obtained by partial dissolutionunder dissolution conditions which will be described later and a bulkcontent of a rare earth atom obtained by total dissolution under thedissolution conditions which will be described later, that is, “surfacelayer portion content/bulk content” may be, for example, 10.0 or less,9.0 or less, 8.0 or less, 7.0 or less, 6.0 or less, 5.0 or less, or 4.0or less. Here, in the hexagonal strontium ferrite powder having a rareearth atom surface layer portion uneven distribution property, the rareearth atoms may be unevenly distributed in the surface layer portion ofa particle configuring the hexagonal strontium ferrite powder, and the“surface layer portion content/bulk content” is not limited to theillustrated upper limit or lower limit.

The partial dissolution and the total dissolution of the hexagonalstrontium ferrite powder will be described below. For the hexagonalstrontium ferrite powder that exists as a powder, the partially andtotally dissolved sample powder is taken from the same lot of powder. Onthe other hand, for the hexagonal strontium ferrite powder included inthe magnetic layer of the magnetic tape, a part of the hexagonalstrontium ferrite powder taken out from the magnetic layer is subjectedto partial dissolution, and the other part is subjected to totaldissolution. The hexagonal strontium ferrite powder can be taken outfrom the magnetic layer by a method described in a paragraph 0032 ofJP2015-091747A, for example. The partial dissolution means thatdissolution is performed such that, at the end of dissolution, theresidue of the hexagonal strontium ferrite powder can be visuallychecked in the solution. For example, by partial dissolution, it ispossible to dissolve a region of 10 to 20 mass % of the particleconfiguring the hexagonal strontium ferrite powder with the totalparticle being 100 mass %. On the other hand, the total dissolutionmeans that dissolution is performed such that, at the end ofdissolution, the residue of the hexagonal strontium ferrite powdercannot be visually checked in the solution.

The partial dissolution and measurement of the surface layer portioncontent are performed by the following method, for example. Here, thefollowing dissolution conditions such as an amount of sample powder areillustrative, and dissolution conditions for partial dissolution andtotal dissolution can be employed in any manner.

A container (for example, a beaker) containing 12 mg of sample powderand 10 mL of 1 mol/L hydrochloric acid is held on a hot plate at a settemperature of 70° C. for 1 hour. The obtained solution is filtered by amembrane filter of 0.1 μm. Elemental analysis of the filtrated solutionis performed by an inductively coupled plasma (ICP) analyzer. In thisway, the surface layer portion content of a rare earth atom with respectto 100 at % of an iron atom can be obtained. In a case where a pluralityof types of rare earth atoms are detected by elemental analysis, thetotal content of all rare earth atoms is defined as the surface layerportion content. This also applies to the measurement of the bulkcontent.

On the other hand, the total dissolution and measurement of the bulkcontent are performed by the following method, for example.

A container (for example, a beaker) containing 12 mg of sample powderand 10 mL of 4 mol/L hydrochloric acid is held on a hot plate at a settemperature of 80° C. for 3 hours. Thereafter, the method is carried outin the same manner as the partial dissolution and the measurement of thesurface layer portion content, and the bulk content with respect to 100at % of an iron atom can be obtained.

From a viewpoint of increasing the reproducing output in a case ofreproducing information recorded on the magnetic tape, it is desirablethat mass magnetization σs of the ferromagnetic powder included in themagnetic tape is high. In this regard, the hexagonal strontium ferritepowder including a rare earth atom but not having the rare earth atomsurface layer portion uneven distribution property tends to have σslargely lower than the hexagonal strontium ferrite powder including norare earth atom. On the other hand, it is considered that hexagonalstrontium ferrite powder having a rare earth atom surface layer portionuneven distribution property is preferable in suppressing such a largedecrease in σs. In an aspect, σs of the hexagonal strontium ferritepowder may be 45 A·m²/kg or more, and may be 47 A·m²/kg or more. On theother hand, from a viewpoint of noise reduction, σs is preferably 80A·m²/kg or less and more preferably 60 A·m²/kg or less. σs can bemeasured using a known measuring device, such as a vibrating samplemagnetometer, capable of measuring magnetic properties. In the presentinvention and this specification, unless otherwise noted, the massmagnetization σs is a value measured at a magnetic field intensity of 15kOe. 1[kOe]=10⁶/4π[A/m]

Regarding the content (bulk content) of a constituent atom of thehexagonal strontium ferrite powder, a strontium atom content may be, forexample, in a range of 2.0 to 15.0 at % with respect to 100 at % of aniron atom. In an aspect, in the hexagonal strontium ferrite powder, adivalent metal atom included in the powder may be only a strontium atom.In another aspect, the hexagonal strontium ferrite powder may includeone or more other divalent metal atoms in addition to a strontium atom.For example, a barium atom and/or a calcium atom may be included. In acase where another divalent metal atom other than a strontium atom isincluded, a barium atom content and a calcium atom content in thehexagonal strontium ferrite powder are, for example, in a range of 0.05to 5.0 at % with respect to 100 at % of an iron atom, respectively.

As a crystal structure of hexagonal ferrite, a magnetoplumbite type(also called an “M type”), a W type, a Y type, and a Z type are known.The hexagonal strontium ferrite powder may have any crystal structure.The crystal structure can be checked by X-ray diffraction analysis. Inthe hexagonal strontium ferrite powder, a single crystal structure ortwo or more crystal structures may be detected by X-ray diffractionanalysis. For example, according to an aspect, in the hexagonalstrontium ferrite powder, only the M-type crystal structure may bedetected by X-ray diffraction analysis. For example, M-type hexagonalferrite is represented by a composition formula of AFe₁₂O₁₉. Here, Arepresents a divalent metal atom, and in a case where the hexagonalstrontium ferrite powder is the M-type, A is only a strontium atom (Sr),or in a case where, as A, a plurality of divalent metal atoms areincluded, as described above, a strontium atom (Sr) accounts for themost on an at % basis. The divalent metal atom content of the hexagonalstrontium ferrite powder is usually determined by the type of crystalstructure of the hexagonal ferrite and is not particularly limited. Thesame applies to the iron atom content and the oxygen atom content. Thehexagonal strontium ferrite powder may include at least an iron atom, astrontium atom, and an oxygen atom, and may further include a rare earthatom. Furthermore, the hexagonal strontium ferrite powder may or may notinclude atoms other than these atoms. As an example, the hexagonalstrontium ferrite powder may include an aluminum atom (Al). A content ofan aluminum atom can be, for example, 0.5 to 10.0 at % with respect to100 at % of an iron atom. From a viewpoint of further suppressing adecrease in reproducing output in repeated reproduction, the hexagonalstrontium ferrite powder includes an iron atom, a strontium atom, anoxygen atom, and a rare earth atom, and the content of atoms other thanthese atoms is preferably 10.0 at % or less, more preferably in a rangeof 0 to 5.0 at %, and may be 0 at % with respect to 100 at % of an ironatom. That is, in an aspect, the hexagonal strontium ferrite powder maynot include atoms other than an iron atom, a strontium atom, an oxygenatom, and a rare earth atom. The content expressed in at % is obtainedby converting a content of each atom (unit: mass %) obtained by totallydissolving hexagonal strontium ferrite powder into a value expressed inat % using an atomic weight of each atom. Further, in the presentinvention and this specification, “not include” for a certain atom meansthat a content measured by an ICP analyzer after total dissolution is 0mass %. A detection limit of the ICP analyzer is usually 0.01 parts permillion (ppm) or less on a mass basis. The “not included” is used as ameaning including that an atom is included in an amount less than thedetection limit of the ICP analyzer. In an aspect, the hexagonalstrontium ferrite powder may not include a bismuth atom (Bi).

In a case where the magnetic tape includes hexagonal strontium ferritepowder in the magnetic layer, the anisotropy magnetic field Hk of themagnetic layer is preferably 14 kOe or more, more preferably 16 kOe ormore, and still more preferably, 18 kOe or more. In addition, theanisotropy magnetic field Hk of the magnetic layer is preferably 90 kOeor less, more preferably 80 kOe or less, and still more preferably 70kOe or less.

The anisotropy magnetic field Hk in the present invention and thisspecification refers to a magnetic field in which magnetization issaturated in a case where a magnetic field is applied in a magnetizationhard axis direction. The anisotropy magnetic field Hk can be measuredusing a known measuring device, such as a vibrating sample magnetometer,capable of measuring magnetic properties. In the magnetic layerincluding hexagonal strontium ferrite powder, the magnetization hardaxis direction of the magnetic layer is an in-plane direction.

ε-Iron Oxide Powder

In the present invention and this specification, “ε-iron oxide powder”refers to ferromagnetic powder in which a ε-iron oxide type crystalstructure is detected as a main phase by X-ray diffraction analysis. Forexample, in a case where the highest intensity diffraction peak isattributed to a ε-iron oxide type crystal structure in an X-raydiffraction spectrum obtained by X-ray diffraction analysis, it isdetermined that the ε-iron oxide type crystal structure is detected asthe main phase. As a manufacturing method of the ε-iron oxide powder, amanufacturing method from a goethite, a reverse micelle method, and thelike are known. All of the manufacturing methods are well known.Regarding a method of manufacturing ε-iron oxide powder in which a partof Fe is substituted with substitutional atoms such as Ga, Co, Ti, Al,or Rh, a description disclosed in J. Jpn. Soc. Powder Metallurgy Vol. 61Supplement, No. 51, pp. 5280 to 5284, J. Mater. Chem. C, 2013, 1, pp.5200 to 5206 can be referred to, for example. Here, the manufacturingmethod of ε-iron oxide powder capable of being used as the ferromagneticpowder in the magnetic layer of the magnetic tape is not limited to themethods described here.

An index for reducing thermal fluctuation, in other words, improvingthermal stability may include an anisotropy constant Ku. The ε-ironoxide powder preferably has Ku of 3.0×10⁴ J/m³ or more, and morepreferably has Ku of 8.0×10⁴ J/m³ or more. Ku of the ε-iron oxide powdermay be, for example, 3.0×10⁵ J/m³ or less. Here, it means that thehigher Ku is, the higher thermal stability is, this is preferable, andthus, a value thereof is not limited to the values exemplified above.

From a viewpoint of increasing the reproducing output in a case ofreproducing information recorded on the magnetic tape, it is desirablethat mass magnetization σs of the ferromagnetic powder included in themagnetic tape is high. In this regard, in an aspect, σs of the ε-ironoxide powder may be 8 A·m²/kg or more, and may be 12 A·m²/kg or more. Onthe other hand, from a viewpoint of noise reduction, σs of the ε-ironoxide powder is preferably 40 A·m²/kg or less and more preferably 35A·m²/kg or less.

In a case where the magnetic tape includes ε-iron oxide powder in themagnetic layer, the anisotropy magnetic field Hk of the magnetic layeris preferably 18 kOe or more, more preferably 30 kOe or more, and stillmore preferably, 38 kOe or more. In addition, the anisotropy magneticfield Hk of the magnetic layer is preferably 100 kOe or less, morepreferably 90 kOe or less, and still more preferably 75 kOe or less. Inthe magnetic layer including ε-iron oxide powder, the magnetization hardaxis direction of the magnetic layer is an in-plane direction.

In the present invention and this specification, unless otherwise noted,an average particle size of various types of powder is a value measuredby the following method using a transmission electron microscope.

The powder is imaged at a magnification ratio of 100,000 with atransmission electron microscope, and the image is printed on printingpaper, is displayed on a display, or the like so that the totalmagnification ratio becomes 500,000 to obtain an image of particlesconfiguring the powder. For powder included in a magnetic layer of amagnetic recording medium, imaging is performed by using a cut pieceproduced by the following method and an image of particles can beobtained. The magnetic recording medium is bonded to a resin block orthe like, a cut piece is produced using a microtome or the like, theproduced cutting piece is observed using a transmission electronmicroscope, and a magnetic layer portion is specified and imaged. Forexample, for a magnetic tape, the magnetic tape can be cut in alongitudinal direction to produce a cut piece.

A target particle is selected from the obtained image of particles, anoutline of the particle is traced with a digitizer, and a size of theparticle (primary particle) is measured. The primary particle is anindependent particle which is not aggregated.

The measurement described above is performed regarding 500 particlesrandomly extracted. An arithmetic average of the particle sizes of 500particles obtained as described above is an average particle size of thepowder. As the transmission electron microscope, a transmission electronmicroscope H-9000 manufactured by Hitachi, Ltd. can be used, forexample. In addition, the measurement of the particle size can beperformed by well-known image analysis software, for example, imageanalysis software KS-400 manufactured by Carl Zeiss. The averageparticle size shown in examples which will be described later is a valuemeasured by using a transmission electron microscope H-9000 manufacturedby Hitachi, Ltd. as the transmission electron microscope, and imageanalysis software KS-400 manufactured by Carl Zeiss as the imageanalysis software, unless otherwise noted, and the average particlevolume of the ferromagnetic powder is a value calculated as a sphereequivalent volume from such an average particle size. In the presentinvention and this specification, the powder means an aggregate of aplurality of particles. For example, ferromagnetic powder means anaggregate of a plurality of ferromagnetic particles. Further, theaggregate of the plurality of particles not only includes an aspect inwhich particles configuring the aggregate directly come into contactwith each other, but also includes an aspect in which a binding agent oran additive which will be described later is interposed between theparticles. The term “particle” is used to describe powder in some cases.

As a method of taking sample powder from the magnetic tape in order tomeasure the particle size, a method disclosed in a paragraph of 0015 ofJP2011-048878A can be used, for example.

In the present invention and this specification, unless otherwise noted,(1) in a case where the shape of the particle observed in the particleimage described above is a needle shape, a fusiform shape, or a columnarshape (here, a height is greater than a maximum long diameter of abottom surface), the size (particle size) of the particles configuringthe powder is shown as a length of a long axis configuring the particle,that is, a long axis length, (2) in a case where the shape of theparticle is a plate shape or a columnar shape (here, a thickness or aheight is smaller than a maximum long diameter of a plate surface or abottom surface), the particle size is shown as a maximum long diameterof the plate surface or the bottom surface, and (3) in a case where theshape of the particle is a sphere shape, a polyhedron shape, or anunspecified shape, and the long axis configuring the particles cannot bespecified from the shape, the particle size is shown as an equivalentcircle diameter. The equivalent circle diameter is a value obtained by acircle projection method.

In addition, regarding an average acicular ratio of the powder, a lengthof a short axis, that is, a short axis length of the particles ismeasured in the measurement described above, a value of (long axislength/short axis length) of each particle is obtained, and anarithmetic average of the values obtained regarding 500 particles iscalculated. Here, unless otherwise noted, in a case of (1), the shortaxis length as the definition of the particle size is a length of ashort axis configuring the particle, in a case of (2), the short axislength is a thickness or a height, and in a case of (3), the long axisand the short axis are not distinguished, thus, the value of (long axislength/short axis length) is assumed as 1, for convenience.

In addition, unless otherwise noted, in a case where the shape of theparticle is specified, for example, in a case of definition of theparticle size (1), the average particle size is an average long axislength, and in a case of the definition (2), the average particle sizeis an average plate diameter. In a case of the definition (3), theaverage particle size is an average diameter (also referred to as anaverage particle diameter).

The content (filling percentage) of the ferromagnetic powder of themagnetic layer is preferably in a range of 50 to 90 mass % and morepreferably in a range of 60 to 90 mass %. The magnetic layer includesferromagnetic powder, can include a binding agent, and can include oneor more additional additives. A high filling percentage of theferromagnetic powder in the magnetic layer is preferable from aviewpoint of improving the recording density.

Binding Agent and Curing Agent

The above magnetic tape may be a coating type magnetic tape, and mayinclude a binding agent in the magnetic layer. The binding agent is oneor more resins. As the binding agent, various resins usually used as abinding agent pf a coating type magnetic recording medium can be used.For example, as the binding agent, a resin selected from a polyurethaneresin, a polyester resin, a polyamide resin, a vinyl chloride resin, anacrylic resin obtained by copolymerizing styrene, acrylonitrile, ormethyl methacrylate, a cellulose resin such as nitrocellulose, an epoxyresin, a phenoxy resin, and a polyvinylalkylal resin such as polyvinylacetal or polyvinyl butyral can be used alone or a plurality of resinscan be mixed with each other to be used. Among these, a polyurethaneresin, an acrylic resin, a cellulose resin, and a vinyl chloride resinare preferable. The resin may be a homopolymer or a copolymer. Theseresins can be used as the binding agent even in the non-magnetic layerand/or a back coating layer which will be described later.

For the binding agent described above, descriptions disclosed inparagraphs 0028 to 0031 of JP2010-024113A can be referred to. Thecontent of the binding agent of the magnetic layer can be, for example,1.0 to 30.0 parts by mass with respect to 100.0 parts by mass of theferromagnetic powder. An average molecular weight of the resin used asthe binding agent can be, for example, 10,000 or more and 200,000 orless as a weight-average molecular weight. The weight-average molecularweight of the present invention and this specification is a valueobtained by performing polystyrene conversion of a value measured by gelpermeation chromatography (GPC) under the following measurementconditions. The weight-average molecular weight shown in examples of abinding agent which will be described later is a value obtained byperforming polystyrene conversion of a value measured under thefollowing measurement conditions.

GPC device: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8mm inner diameter (ID)×30.0 cm)

Eluent: Tetrahydrofuran (THF)

In addition, a curing agent can also be used together with the resinwhich can be used as the binding agent. As the curing agent, in anaspect, a thermosetting compound which is a compound in which a curingreaction (crosslinking reaction) proceeds due to heating can be used,and in another aspect, a photocurable compound in which a curingreaction (crosslinking reaction) proceeds due to light irradiation canbe used. At least a part of the curing agent can be included in themagnetic layer in a state of being reacted (crosslinked) with othercomponents such as the binding agent, by proceeding the curing reactionin a magnetic layer forming step. The same applies to the layer formedusing this composition in a case where the composition used to form theother layer includes a curing agent. The preferred curing agent is athermosetting compound, and polyisocyanate is suitable for this. Fordetails of the polyisocyanate, descriptions disclosed in paragraphs 0124and 0125 of JP2011-216149A can be referred to. The content of the curingagent in a magnetic layer forming composition can be, for example, 0 to80.0 parts by mass, and can be 50.0 to 80.0 parts by mass from aviewpoint of improving a strength of the magnetic layer, with respect to100.0 parts by mass of the binding agent.

Additive

The magnetic layer may include one or more kinds of additives, asnecessary. As the additives, the curing agent described above is used asan example. In addition, examples of the additive which can be includedin the magnetic layer include non-magnetic powder (for example,inorganic powder or carbon black), a lubricant, a dispersing agent, adispersing assistant, an antibacterial agent, an antistatic agent, andan antioxidant. For example, for the lubricant, descriptions disclosedin paragraphs 0030 to 0033, 0035, and 0036 of JP2016-126817A can bereferred to. The non-magnetic layer described later may include alubricant. For the lubricant which may be included in the non-magneticlayer, descriptions disclosed in paragraphs 0030, 0031, 0034, 0035, and0036 of JP2016-126817A can be referred to. For the dispersing agent,descriptions disclosed in paragraphs 0061 and 0071 of JP2012-133837A canbe referred to. A dispersing agent may be added to a non-magnetic layerforming composition. For the dispersing agent which can be included inthe non-magnetic layer forming composition, a description disclosed in aparagraph 0061 of JP2012-133837A can be referred to. As the non-magneticpowder that can be included in the magnetic layer, non-magnetic powderwhich can function as an abrasive, or non-magnetic powder which canfunction as a projection formation agent which forms projectionssuitably protruded from the magnetic layer surface (for example,non-magnetic colloidal particles) is used. In addition, carbon black canbe contained in the magnetic layer. For example, carbon black having anaverage particle size of 5 to 300 nm can be used. The carbon blackcontent of the magnetic layer can be, for example, 0.1 to 30.0 parts bymass per 100.0 parts by mass of the ferromagnetic powder. In an aspect,the surface shape of the magnetic layer can be controlled by adjustingthe carbon black content of the magnetic layer. As the additive, acommercially available product can be suitably selected or manufacturedby a well-known method according to the desired properties, and anyamount thereof can be used. Examples of the additive that can be used toimprove the dispersibility of the abrasive in the magnetic layercontaining the abrasive include a dispersing agent disclosed inparagraphs 0012 to 0022 of JP2013-131285A.

The magnetic layer described above can be provided directly on a surfaceof the non-magnetic support or indirectly through the non-magneticlayer.

Non-Magnetic Layer

Next, the non-magnetic layer will be described. The above magnetic tapemay have a magnetic layer directly on the surface of the non-magneticsupport, or may have a magnetic layer on the surface of the non-magneticsupport via a non-magnetic layer including non-magnetic powder. Anaverage particle size of the non-magnetic powder is preferably in arange of 5 to 500 nm, and more preferably in a range of 10 to 200 nm.Non-magnetic powder used for the non-magnetic layer may be an inorganicpowder or an organic powder. In addition, carbon black and the like canbe used. Examples of the inorganic powder include powder such as metal,metal oxide, metal carbonate, metal sulfate, metal nitride, metalcarbide, and metal sulfide. The non-magnetic powder can be purchased asa commercially available product or can be manufactured by a well-knownmethod. For details thereof, descriptions disclosed in paragraphs 0146to 0150 of JP2011-216149A can be referred to. For carbon black which canbe used in the non-magnetic layer, descriptions disclosed in paragraphs0040 and 0041 of JP2010-024113A can be referred to. The content (fillingpercentage) of the non-magnetic powder of the non-magnetic layer ispreferably in a range of 50 to 90 mass % and more preferably in a rangeof 60 to 90 mass %.

The non-magnetic layer includes non-magnetic powder, and may include abinding agent together with the non-magnetic powder. In regards to otherdetails of a binding agent or an additive of the non-magnetic layer, awell-known technology regarding the non-magnetic layer can be applied.In addition, in regards to the type and the content of the bindingagent, and the type and the content of the additive, for example, awell-known technology regarding the magnetic layer can be applied.

In the present invention and this specification, the non-magnetic layeralso includes a substantially non-magnetic layer including a smallamount of ferromagnetic powder as impurities, for example, orintentionally, together with the non-magnetic powder. Here, thesubstantially non-magnetic layer is a layer having a residual magneticflux density equal to or smaller than 10 mT, a layer having a coercivityequal to or smaller than 100 Oe, or a layer having a residual magneticflux density equal to or smaller than 10 mT and a coercivity equal to orsmaller than 100 Oe. It is preferable that the non-magnetic layer doesnot have a residual magnetic flux density and a coercivity.

Back Coating Layer

The above magnetic tape includes a back coating layer includingnon-magnetic powder on a surface side of a non-magnetic support oppositeto a surface side provided with a magnetic layer. Preferably, the backcoating layer includes one or both of carbon black and inorganic powder.For example, the surface shape of the back coating layer can becontrolled by using non-magnetic powders having different particle sizesas the non-magnetic powder for the back coating layer. For example, thenumber of the above protrusions can be controlled by using carbon blackhaving an average particle size of 15 to 50 nm (hereinafter, referred toas “fine particle carbon black”.) and carbon black having an averageparticle size of 75 to 500 nm (hereinafter, referred to as “coarseparticle carbon black”.) in combination and adjusting a mixing ratio ofboth carbon blacks. The content of the carbon black in the back coatinglayer is preferably in a range of 50.0 to 200.0 parts by mass, and morepreferably in a range of 75.0 to 150.0 with respect to 100.0 parts bymass of the binding agent.

The back coating layer may include one or more inorganic powders,preferably with carbon black. A mixing ratio of the inorganic powder andthe carbon black is preferably 9:1 to 7:3 as inorganic powder:carbonblack (mass basis). As an inorganic powder, inorganic powder having anaverage particle size of 80 to 250 nm and a Mohs hardness of 5 to 9 canbe used, for example. As an inorganic powder, non-magnetic powdergenerally used for a non-magnetic layer, non-magnetic powder generallyused as an abrasive for a magnetic layer, or the like can be used, amongthese, α-iron oxide, α-alumina, or the like is preferable. The contentof the inorganic powder in the back coating layer is preferably in arange of 300.0 to 700.0 parts by mass, and more preferably in a range of400.0 to 500.0 with respect to 100.0 parts by mass of the binding agent.

The back coating layer includes non-magnetic powder, can include abinding agent, and can also include one or more additives. In regards tothe binding agent and the additive of the back coating layer, thewell-known technique regarding the back coating layer can be applied,and the well-known technique regarding the treatment of the magneticlayer and/or the non-magnetic layer can be applied. For example, for theback coating layer, descriptions disclosed in paragraphs 0018 to 0020 ofJP2006-331625A and page 4, line 65 to page 5, line 38 of U.S. Pat. No.7,029,774B can be referred to.

Non-Magnetic Support

Next, the non-magnetic support will be described. As the non-magneticsupport (hereinafter, also simply referred to as a “support”),well-known components such as polyethylene terephthalate, polyethylenenaphthalate, polyamide, polyamide imide, and aromatic polyamidesubjected to biaxial stretching are used. Among these, polyethyleneterephthalate, polyethylene naphthalate, and polyamide are preferable.Corona discharge, plasma treatment, easy-bonding treatment, or heattreatment may be performed with respect to these supports in advance. Byusing a support having a different PSD on a surface of a side on whichthe magnetic layer is formed and/or a surface of a side on which theback coating layer is formed, the surface shape of the magnetic layerand/or the surface shape of the back coating layer can be changed. A PSDat a 5 μm pitch on the surface of the side of the support on which themagnetic layer is formed (hereinafter, referred to as a “magneticsurface side 5 μm PSD”.) can be, for example, in a range of 2.00 E+02 to1.20 E+04 nm³, and a PSD at a 10 μm pitch on the surface of the side ofthe support on which the back coating layer is formed (hereinafter,referred to as a “back surface side 10 μm PSD”.) can be, for example, ina range of 5.00 E+02 to 3.00 E+06 nm³. In a case where the support ismanufactured by a well-known method, the surface shape of the supportcan be adjusted according to the size and the content of thenon-magnetic powder contained in the support. In addition, by forming asmoothing layer on one side or both sides of the support by using aradiation curable resin, the surface shape of the surface on which themagnetic layer or the back coating layer is formed thereon can beadjusted.

Various Thicknesses

A thickness of the non-magnetic support is, for example, 3.0 to 50.0 μm,preferably 3.0 to 10.0 μm, and more preferably 3.0 to 5.0 μm.

A thickness of the magnetic layer can be optimized according to asaturation magnetization amount of the used magnetic head, a head gaplength, a band of a recording signal, and the like, is generally 10 to150 nm, and, from a viewpoint of high density recording, is preferablyin the range of 20 to 120 nm and more preferably in the range of 30 to100 nm. The magnetic layer may be at least a single layer, the magneticlayer may be separated into two or more layers having different magneticproperties, and a configuration of a well-known multilayered magneticlayer can be applied as the magnetic layer. A thickness of the magneticlayer in a case where the magnetic layer is separated into two or morelayers is a total thickness of the layers.

A thickness of the non-magnetic layer is, for example, 0.1 to 1.5 μm,and preferably 0.1 to 1.0 μm.

A thickness of the back coating layer is preferably 0.9 μm or less, andmore preferably 0.1 to 0.7 μm.

Thicknesses of each layer of the magnetic tape and the non-magneticsupport can be obtained by a well-known film thickness measurementmethod. As an example, a cross section of the magnetic tape in athickness direction is exposed by known means such as an ion beam or amicrotome, and then a cross section observation is performed using ascanning electron microscope in the exposed cross section, for example.In the cross section observation, various thicknesses can be obtained asa thickness obtained at one portion of the cross section, or anarithmetic average of thicknesses obtained at a plurality of portions oftwo or more portions, for example, two portions which are randomlyextracted. In addition, the thickness of each layer may be obtained as adesigned thickness calculated according to manufacturing conditions.

Manufacturing Step

A composition for forming the magnetic layer, the non-magnetic layer,and the back coating layer usually contains a solvent together with thevarious components described above. As a solvent, various organicsolvents generally used for manufacturing a coating type magneticrecording medium can be used. The amount of the solvent in each layerforming composition is not particularly limited, and can be the same asthat of each layer forming composition of a normal coating type magneticrecording medium. A step of preparing each layer forming composition cangenerally include at least a kneading step, a dispersing step, and amixing step provided before and after these steps as necessary. Eachstep may be divided into two or more stages. Various components used forthe preparation of each layer forming composition may be added at aninitial stage or in a middle stage of each step. In addition, eachcomponent may be separately added in two or more steps.

In order to prepare each layer forming composition, a well-knowntechnique can be used. In the kneading step, preferably, a kneaderhaving a strong kneading force such as an open kneader, a continuouskneader, a pressure kneader, or an extruder is used. Details of thekneading processes are described in JP1989-106338A (JP-H01-106338A) andJP1989-079274A (JP-H01-079274A). Moreover, in order to disperse eachlayer forming composition, one or more kinds of dispersed beads selectedfrom the group consisting of glass beads and other dispersed beads canbe used as a dispersion medium. As such dispersed beads, zirconia beads,titania beads, and steel beads which are dispersed beads having a highspecific gravity are suitable. These dispersed beads can be used byoptimizing the particle diameter (bead diameter) and filling rate. As adispersing device, a well-known dispersing device can be used. Eachlayer forming composition may be filtered by a well-known method, beforesubjecting to a coating step. The filtering can be performed by using afilter, for example. As the filter used in the filtering, a filterhaving a pore diameter of 0.01 to 3 μm (for example, filter made ofglass fiber or filter made of polypropylene) can be used, for example.As coarse aggregates are removed by filtration, the waviness componentand/or the protrusion on the surface of the magnetic layer and/or theback coating layer tends to be reduced.

The magnetic layer can be formed, for example, by directly coating themagnetic layer forming composition onto the non-magnetic support orperforming multilayer coating of the magnetic layer forming compositionwith the non-magnetic layer forming composition in order or at the sametime. The back coating layer can be formed by coating the back coatinglayer forming composition to a side of the non-magnetic support oppositeto a side provided with the magnetic layer (or to be provided with themagnetic layer). For details of coating for forming each layer, adescription disclosed in a paragraph 0051 of JP2010-024113A can bereferred to.

After the coating step, various processes such as a drying process, anorientation process of the magnetic layer, and a surface smoothingprocess (calendering process) can be performed. For various processes,for example, well-known techniques disclosed in paragraphs 0052 to 0057of JP2010-024113A can be referred to. For example, the surface shape ofeach layer can be controlled also by drying conditions (temperature andthe like). In addition, for example, a coating layer of the magneticlayer forming composition can be subjected to an orientation processwhile the coating layer is in a wet (undried) state. For the orientationprocess, the various well-known technologies such as descriptionsdisclosed in a paragraph 0067 of JP2010-231843A can be used. Forexample, a vertical orientation process can be performed by a well-knownmethod such as a method using a polar opposing magnet. In an orientationzone, a drying speed of the coating layer can be controlled depending ona temperature and a flow rate of dry air and/or a transportation speedof the magnetic tape in the orientation zone. In addition, the coatinglayer may be preliminarily dried before the transportation to theorientation zone. For the calendering process, in a case where acalendering condition is strengthened, the waviness component and/orirregularities on the surface of the magnetic layer and/or the backcoating layer tends to be reduced. Examples of the calendering conditioninclude a calendering pressure, a calendering temperature (a surfacetemperature of a calendering roll), a calendering speed, a hardness of acalendering roll, and the like. As values of the calendering pressure,the calendering temperature, and the hardness of a calendering roll areincreased, the calendering process is strengthened, and as a value ofthe calendering speed is decreased, the calendering process isstrengthened. Further, while a coating layer of a composition forforming any layer is in a wet (undried) state, the coating layer can besheared by performing a smoothing process by a well-known method. Byapplying a shear force to the coating layer, the waviness componentand/or the protrusion of the magnetic layer and/or the back coatinglayer tends to be reduced.

It is possible to form a servo pattern in the manufactured magnetic tapeby a known method in order to enable tracking control of the magnetichead in the magnetic recording and reproducing apparatus, control of arunning speed of the magnetic tape, and the like. The “formation ofservo pattern” can also be referred to as “recording of servo signal”.Hereinafter, the formation of the servo pattern will be described.

The servo pattern is usually formed along a longitudinal direction ofthe magnetic tape. Examples of control (servo control) types using aservo signal include a timing-based servo (TBS), an amplitude servo, anda frequency servo.

As shown in a european computer manufacturers association (ECMA)-319, amagnetic tape (generally called “LTO tape”) conforming to a lineartape-open (LTO) standard employs a timing-based servo type. In thistiming-based servo type, the servo pattern is formed by continuouslydisposing a plurality of pairs of non-parallel magnetic stripes (alsoreferred to as “servo stripes”) in a longitudinal direction of themagnetic tape. As described above, the reason why the servo pattern isformed of a pair of non-parallel magnetic stripes is to indicate, to aservo signal reading element passing over the servo pattern, a passingposition thereof. Specifically, the pair of magnetic stripes is formedso that an interval thereof continuously changes along a width directionof the magnetic tape, and the servo signal reading element reads theinterval to thereby sense a relative position between the servo patternand the servo signal reading element. Information on this relativeposition enables tracking on a data track. Therefore, a plurality ofservo tracks are usually set on the servo pattern along a widthdirection of the magnetic tape.

A servo band is formed of servo signals continuous in a longitudinaldirection of the magnetic tape. A plurality of servo bands are usuallyprovided on the magnetic tape. For example, in an LTO tape, the numberis five. A region interposed between two adjacent servo bands isreferred to as a data band. The data band is formed of a plurality ofdata tracks, and each data track corresponds to each servo track.

Further, in an aspect, as shown in JP2004-318983A, informationindicating a servo band number (referred to as “servo bandidentification (ID)” or “unique data band identification method (UDIM)information”) is embedded in each servo band. This servo band ID isrecorded by shifting a specific one of the plurality of pairs of theservo stripes in the servo band so that positions thereof are relativelydisplaced in a longitudinal direction of the magnetic tape.Specifically, a way of shifting the specific one of the plurality ofpairs of servo stripes is changed for each servo band. Accordingly, therecorded servo band ID is unique for each servo band, and thus, theservo band can be uniquely specified only by reading one servo band witha servo signal reading element.

Incidentally, as a method for uniquely specifying the servo band, thereis a method using a staggered method as shown in ECMA-319. In thisstaggered method, a group of pairs of non-parallel magnetic stripes(servo stripes) disposed continuously in plural in a longitudinaldirection of the magnetic tape is recorded so as to be shifted in alongitudinal direction of the magnetic tape for each servo band. Sincethis combination of shifting methods between adjacent servo bands isunique throughout the magnetic tape, it is possible to uniquely specifya servo band in a case of reading a servo pattern with two servo signalreading element elements.

As shown in ECMA-319, information indicating a position of the magnetictape in the longitudinal direction (also referred to as “longitudinalposition (LPOS) information”) is usually embedded in each servo band.This LPOS information is also recorded by shifting the positions of thepair of servo stripes in the longitudinal direction of the magnetictape, as the UDIM information. Here, unlike the UDIM information, inthis LPOS information, the same signal is recorded in each servo band.

It is also possible to embed, in the servo band, the other informationdifferent from the above UDIM information and LPOS information. In thiscase, the embedded information may be different for each servo band asthe UDIM information or may be common to all servo bands as the LPOSinformation.

As a method of embedding information in the servo band, it is possibleto employ a method other than the above. For example, a predeterminedcode may be recorded by thinning out a predetermined pair from the groupof pairs of servo stripes.

A head for forming a servo pattern is called a servo write head. Theservo write head has a pair of gaps corresponding to the pair ofmagnetic stripes as many as the number of servo bands. Usually, a coreand a coil are connected to each pair of gaps, and by supplying acurrent pulse to the coil, a magnetic field generated in the core cancause generation of a leakage magnetic field in the pair of gaps. In acase of forming the servo pattern, by inputting a current pulse whilerunning the magnetic tape on the servo write head, the magnetic patterncorresponding to the pair of gaps is transferred to the magnetic tape toform the servo pattern. A width of each gap can be appropriately setaccording to a density of the servo pattern to be formed. The width ofeach gap can be set to, for example, 1 μm or less, 1 to 10 μm, 10 μm ormore, and the like.

Before the servo pattern is formed on the magnetic tape, the magnetictape is usually subjected to a demagnetization (erasing) process. Thiserasing process can be performed by applying a uniform magnetic field tothe magnetic tape using a direct current magnet or an alternatingcurrent magnet. The erasing process includes direct current (DC) erasingand alternating current (AC) erasing. AC erasing is performed bygradually decreasing an intensity of the magnetic field while reversinga direction of the magnetic field applied to the magnetic tape. On theother hand, DC erasing is performed by applying a unidirectionalmagnetic field to the magnetic tape. As the DC erasing, there are twomethods. A first method is horizontal DC erasing of applying a magneticfield in one direction along a longitudinal direction of the magnetictape. A second method is vertical DC erasing of applying a magneticfield in one direction along a thickness direction of the magnetic tape.The erasing process may be performed on the entire magnetic tape or maybe performed for each servo band of the magnetic tape.

A direction of the magnetic field of the servo pattern to be formed isdetermined according to a direction of the erasing. For example, in acase where the horizontal DC erasing is performed to the magnetic tape,the servo pattern is formed so that the direction of the magnetic fieldis opposite to the direction of the erasing. Therefore, an output of aservo signal obtained by reading the servo pattern can be increased. Asshown in JP2012-053940A, in a case where a magnetic pattern istransferred to, using the gap, a magnetic tape that has been subjectedto vertical DC erasing, a servo signal obtained by reading the formedservo pattern has a monopolar pulse shape. On the other hand, in a casewhere a magnetic pattern is transferred to, using the gap, a magnetictape that has been subjected to horizontal DC erasing, a servo signalobtained by reading the formed servo pattern has a bipolar pulse shape.

The magnetic tape is usually accommodated in a magnetic tape cartridgeand the magnetic tape cartridge is mounted in the magnetic recording andreproducing apparatus.

Magnetic Tape Cartridge

Another aspect of the present invention relates to a magnetic tapecartridge including the magnetic tape described above.

The details of the magnetic tape included in the above magnetic tapecartridge are as described above.

In the magnetic tape cartridge, generally, the magnetic tape isaccommodated inside a cartridge body in a state of being wound around areel. The reel is rotatably provided inside the cartridge body. As themagnetic tape cartridge, a single reel type magnetic tape cartridgehaving one reel inside the cartridge body and a dual reel type magnetictape cartridge having two reels inside the cartridge body are widelyused. In a case where the single reel type magnetic tape cartridge ismounted on a magnetic recording and reproducing apparatus for recordingand/or reproducing data on the magnetic tape, the magnetic tape ispulled out of the magnetic tape cartridge to be wound around the reel onthe magnetic recording and reproducing apparatus side. A magnetic headis disposed on a magnetic tape transportation path from the magnetictape cartridge to a winding reel. Feeding and winding of the magnetictape are performed between a reel (supply reel) on the magnetic tapecartridge side and a reel (winding reel) on the magnetic recording andreproducing apparatus side. During this time, data is recorded and/orreproduced as the magnetic head and the magnetic layer surface of themagnetic tape come into contact with each other to be slid on eachother. With respect to this, in the dual reel type magnetic tapecartridge, both reels of the supply reel and the winding reel areprovided in the magnetic tape cartridge. The above magnetic tapecartridge may be either a single reel type or a dual reel type magnetictape cartridge. The above magnetic tape cartridge has only to includethe magnetic tape according to an aspect of the present invention, andthe well-known technology can be applied to the others.

Magnetic Recording and Reproducing Apparatus

Another aspect of the present invention relates to a magnetic recordingand reproducing apparatus including the magnetic tape described aboveand a magnetic head.

In the present invention and this specification, the “magnetic recordingand reproducing apparatus” means an apparatus capable of performing atleast one of the recording of data on the magnetic tape or thereproducing of data recorded on the magnetic tape. Such an apparatus isgenerally called a drive. The magnetic recording and reproducingapparatus can be a sliding type magnetic recording and reproducingapparatus. The sliding type magnetic recording and reproducing apparatusis an apparatus in which the magnetic layer surface and the magnetichead come into contact with each other to be slid on each other, in acase of performing the recording of data on the magnetic tape and/orreproducing of the recorded data.

The magnetic head included in the magnetic recording and reproducingapparatus can be a recording head capable of performing the recording ofdata on the magnetic tape, or can be a reproducing head capable ofperforming the reproducing of data recorded on the magnetic tape. Inaddition, in an aspect, the magnetic recording and reproducing apparatuscan include both of a recording head and a reproducing head as separatemagnetic heads. In another aspect, the magnetic head included in themagnetic recording and reproducing apparatus can have a configurationthat both of an element for recording data (recording element) and anelement for reproducing data (reproducing element) are included in onemagnetic head. Hereinafter, the element for recording and the elementfor reproducing data are collectively referred to as an “element fordata”. As the reproducing head, a magnetic head (MR head) including amagnetoresistive (MR) element capable of sensitively reading datarecorded on the magnetic tape as a reproducing element is preferable. Asthe MR head, various known MR heads such as an anisotropicmagnetoresistive (AMR) head, a giant magnetoresistive (GMR) head, and atunnel magnetoresistive (TMR) head can be used. In addition, themagnetic head which performs the recording of data and/or thereproducing of data may include a servo signal reading element.Alternatively, as a head other than the magnetic head which performs therecording of data and/or the reproducing of data, a magnetic head (servohead) comprising a servo signal reading element may be included in themagnetic recording and reproducing apparatus. For example, a magnetichead that records data and/or reproduces recorded data (hereinafter alsoreferred to as “recording and reproducing head”) can include two servosignal reading elements, and the two servo signal reading elements canread two adjacent servo bands simultaneously. One or a plurality ofelements for data can be disposed between the two servo signal readingelements.

In the magnetic recording and reproducing apparatus, recording of dataon the magnetic tape and/or reproducing of data recorded on the magnetictape can be performed as the magnetic layer surface of the magnetic tapeand the magnetic head come into contact with each other to be slid oneach other. The magnetic recording and reproducing apparatus has only toinclude the magnetic tape according to an aspect of the presentinvention, and the well-known technology can be applied to the others.

For example, in a case of recording data and/or reproducing the recordeddata, first, tracking using a servo signal is performed. That is, bycausing the servo signal reading element to follow a predetermined servotrack, the element for data is controlled to pass on the target datatrack. Displacement of the data track is performed by changing a servotrack to be read by the servo signal reading element in a tape widthdirection.

The recording and reproducing head can also perform recording and/orreproducing with respect to other data bands. In this case, the servosignal reading element may be displaced to a predetermined servo bandusing the above described UDIM information, and tracking for the servoband may be started.

EXAMPLES

Hereinafter, the present invention will be described with reference toexamples. Here, the present invention is not limited to aspects shown inthe examples. “Parts” and “%” in the following description mean “partsby mass” and “mass %”, unless otherwise noted. The following processesand evaluation were performed in the air of 23° C.±1° C., unlessotherwise specified. “eq” described below is an equivalent and is a unitthat cannot be converted into SI unit.

In Table 1 below, “SrFe1” and “SrFe2” represent hexagonal strontiumferrite powder, “ε-iron oxide” represents ε-iron oxide powder, and“BaFe” represents hexagonal barium ferrite powder.

An average particle volume of various ferromagnetic powders describedbelow is a value obtained by the method described above.

An anisotropy constant Ku is a value obtained for each ferromagneticpowder by the method described above using a vibrating samplemagnetometer (manufactured by Toei Industry Co., Ltd.).

A mass magnetization σs is a value measured at a magnetic fieldintensity of 15 kOe using a vibrating sample magnetometer (manufacturedby Toei Industry Co., Ltd.).

Further, an anisotropy magnetic field Hk of the magnetic layer describedbelow is a value measured using a vibrating sample magnetometer of aTM-VSM5050-SMS type (manufactured by Tamagawa Co., Ltd.).

Method for Manufacturing Ferromagnetic Powder

Manufacturing Method 1 of Hexagonal Strontium Ferrite Powder

1707 g of SrCO₃, 687 g of H₃BO₃, 1120 g of Fe₂O₃, 45 g of Al(OH)₃, 24 gof BaCO₃, 13 g of CaCO₃, and 235 g of Nd₂O₃ were weighed and mixed by amixer to obtain a raw material mixture.

The obtained raw material mixture was melted in a platinum crucible at amelting temperature of 1390° C., and a hot water outlet provided at abottom of the platinum crucible was heated while stirring a melt, andthe melt was discharged in a rod shape at about 6 g/sec. Hot water wasrolled and quenched by a water-cooled twin roller to manufacture anamorphous body.

280 g of the manufactured amorphous body was charged into an electricfurnace, was heated to 635° C. (crystallization temperature) at aheating rate of 3.5° C./min, and was kept at the same temperature for 5hours to precipitate (crystallize) hexagonal strontium ferriteparticles.

Next, a crystallized product obtained above including hexagonalstrontium ferrite particles was coarsely pulverized in a mortar, and1000 g of zirconia beads having a particle diameter of 1 mm and 800 mlof an acetic acid aqueous solution of 1% concentration were added to thecrystallized product in a glass bottle, to be dispersed by a paintshaker for 3 hours. Thereafter, the obtained dispersion liquid wasseparated from the beads, to be put in a stainless beaker. Thedispersion liquid was statically left at a liquid temperature of 100° C.for 3 hours and subjected to a dissolving process of a glass component,and then the crystallized product was sedimented by a centrifugalseparator to be washed by repeatedly performing decantation and wasdried in a heating furnace at an internal temperature of the furnace of110° C. for 6 hours to obtain hexagonal strontium ferrite powder.

An average particle volume of the hexagonal strontium ferrite powderobtained above (“SrFe1” in Table 1 described later) was a valuedescribed in Table 1 described later, an anisotropy constant Ku was2.2×10⁵ J/m³, and a mass magnetization σs was 49 A·m²/kg.

12 mg of sample powder was taken from the hexagonal strontium ferritepowder obtained above, elemental analysis of the filtrated solutionobtained by partially dissolving this sample powder under dissolutionconditions illustrated above was performed by an ICP analyzer, and asurface layer portion content of a neodymium atom was determined.

Separately, 12 mg of sample powder was taken from the hexagonalstrontium ferrite powder obtained above, elemental analysis of thefiltrated solution obtained by completely dissolving this sample powderunder dissolution conditions illustrated above was performed by an ICPanalyzer, and a bulk content of a neodymium atom was determined.

A content (bulk content) of a neodymium atom with respect to 100 at % ofan iron atom in the hexagonal strontium ferrite powder obtained abovewas 2.9 at %. A surface layer portion content of a neodymium atom was8.0 at %. It was confirmed that a ratio between a surface layer portioncontent and a bulk content, that is, “surface layer portion content/bulkcontent” was 2.8, and a neodymium atom was unevenly distributed in asurface layer of a particle.

The fact that the powder obtained above shows a crystal structure ofhexagonal ferrite was confirmed by performing scanning with CuKα raysunder conditions of a voltage of 45 kV and an intensity of 40 mA andmeasuring an X-ray diffraction pattern under the following conditions(X-ray diffraction analysis). The powder obtained above showed a crystalstructure of hexagonal ferrite of a magnetoplumbite type (M type). Acrystal phase detected by X-ray diffraction analysis was a single phaseof a magnetoplumbite type.

PANalytical X'Pert Pro analyzer, PIXcel detector

Soller slit of incident beam and diffracted beam: 0.017 radians

Fixed angle of dispersion slit: ¼ degrees

Mask: 10 mm

Anti-scattering slit: ¼ degrees

Measurement mode: continuous

Measurement time per stage: 3 seconds

Measurement speed: 0.017 degrees per second

Measurement step: 0.05 degrees

Manufacturing Method 2 of Hexagonal Strontium Ferrite Powder 1725 g ofSrCO₃, 666 g of H₃BO₃, 1332 g of Fe₂O₃, 52 g of Al(OH)₃, 34 g of CaCO₃,and 141 g of BaCO₃ were weighed and mixed by a mixer to obtain a rawmaterial mixture.

The obtained raw material mixture was melted in a platinum crucible at amelting temperature of 1380° C., and a hot water outlet provided at abottom of the platinum crucible was heated while stirring a melt, andthe melt was discharged in a rod shape at about 6 g/sec. Hot water wasrolled and quenched by a water-cooled twin roller to manufacture anamorphous body.

280 g of the obtained amorphous body was charged into an electricfurnace, was heated to 645° C. (crystallization temperature), and waskept at the same temperature for 5 hours to precipitate (crystallize)hexagonal strontium ferrite particles.

Next, a crystallized product obtained above including hexagonalstrontium ferrite particles was coarsely pulverized in a mortar, and1000 g of zirconia beads having a particle diameter of 1 mm and 800 mlof an acetic acid aqueous solution of 1% concentration were added to thecrystallized product in a glass bottle, to be dispersed by a paintshaker for 3 hours. Thereafter, the obtained dispersion liquid wasseparated from the beads, to be put in a stainless beaker. Thedispersion liquid was statically left at a liquid temperature of 100° C.for 3 hours and subjected to a dissolving process of a glass component,and then the crystallized product was sedimented by a centrifugalseparator to be washed by repeatedly performing decantation and wasdried in a heating furnace at an internal temperature of the furnace of110° C. for 6 hours to obtain hexagonal strontium ferrite powder.

An average particle volume of the obtained hexagonal strontium ferritepowder (“SrFe2” in Table 1 described later) was a value described inTable 1 described later, an anisotropy constant Ku was 2.0×10⁵ J/m³, anda mass magnetization σs was 50 A·m²/kg.

Method of manufacturing ε-Iron Oxide Powder 8.3 g of iron(III) nitratenonahydrate, 1.3 g of gallium(III) nitrate octahydrate, 190 mg ofcobalt(II) nitrate hexahydrate, 150 mg of titanium(IV) sulfate, and 1.5g of polyvinylpyrrolidone (PVP) were dissolved in 90 g of pure water,and while the dissolved product was stirred using a magnetic stiffer,4.0 g of an aqueous ammonia solution having a concentration of 25% wasadded to the dissolved product under a condition of an atmospheretemperature of 25° C. in an air atmosphere, and the dissolved productwas stirred for 2 hours while maintaining a temperature condition of theatmosphere temperature of 25° C. A citric acid solution obtained bydissolving 1 g of citric acid in 9 g of pure water was added to theobtained solution, and the mixture was stirred for 1 hour. The powdersedimented after stirring was collected by centrifugal separation, waswashed with pure water, and was dried in a heating furnace at a furnacetemperature of 80° C.

800 g of pure water was added to the dried powder, and the powder wasdispersed again in water to obtain dispersion liquid. The obtaineddispersion liquid was heated to a liquid temperature of 50° C., and 40 gof an aqueous ammonia solution having a concentration of 25% wasdropwise added with stirring. After stirring for 1 hour whilemaintaining the temperature at 50° C., 14 mL of tetraethoxysilane (TEOS)was dropwise added and was stirred for 24 hours. Powder sedimented byadding 50 g of ammonium sulfate to the obtained reaction solution wascollected by centrifugal separation, was washed with pure water, and wasdried in a heating furnace at a furnace temperature of 80° C. for 24hours to obtain a ferromagnetic powder precursor.

The obtained ferromagnetic powder precursor was loaded into a heatingfurnace at a furnace temperature of 1000° C. in an air atmosphere andwas heat-treated for 4 hours.

The heat-treated ferromagnetic powder precursor was put into an aqueoussolution of 4 mol/L sodium hydroxide (NaOH), and the liquid temperaturewas maintained at 70° C. and was stirred for 24 hours, whereby a silicicacid compound as an impurity was removed from the heat-treatedferromagnetic powder precursor.

Thereafter, the ferromagnetic powder from which the silicic acidcompound was removed was collected by centrifugal separation, and waswashed with pure water to obtain a ferromagnetic powder.

The composition of the obtained ferromagnetic powder that was checked byhigh-frequency inductively coupled plasma-optical emission spectrometry(ICP-OES) has Ga, Co, and a Ti substitution type ε-iron oxide(ε-Ga_(0.58)Fe_(1.42)O₃). In addition, X-ray diffraction analysis isperformed under the same condition as that described above for themanufacturing method 1 of hexagonal strontium ferrite powder, and from apeak of an X-ray diffraction pattern, it is checked that the obtainedferromagnetic powder does not include α-phase and γ-phase crystalstructures, and has a single-phase and ε-phase crystal structure (ε-ironoxide type crystal structure).

An average particle volume of the obtained ε-iron oxide powder (“ε-ironoxide” in a table described later) was a value described in a tabledescribed later, an anisotropy constant Ku was 1.2×10⁵ J/m³, and a massmagnetization σs was 16 A·m²/kg.

Example 1

List of each layer forming composition is shown below.

List of Magnetic Layer Forming Composition Ferromagnetic powder (seeTable 2) 100.0 parts Polyurethane resin 17.0 parts Branched sidechain-containing polyester polyol/diphenylmethane diisocyanate system,—SO₃Na = 400 eq/ton α-Al₂O₃ (average particle size: 0.15 μm) 5.0 partsDiamond powder (average particle size: 60 nm) 1.0 part Carbon black(average particle size: 20 nm) 1.0 part Cyclohexanone 110.0 parts Methylethyl ketone 100.0 parts Toluene 100.0 parts Butyl stearate 2.0 partsStearic acid 1.0 part

List of Non-Magnetic Layer Forming Composition Non-magnetic inorganicpowder: α-iron oxide 100.0 parts Average particle size: 10 nm Averageacicular ratio: 1.9 Brunauer-emmett-teller (BET) specific surface area:75 m²/g Carbon black (average particle size: 20 nm) 25.0 parts SO₃Nagroup-containing polyurethane resin 18.0 parts (weight-average molecularweight: 70,000, SO₃Na group: 0.2 meq/g) Stearic acid 1.0 partCyclohexanone 300.0 parts Methyl ethyl ketone 300.0 parts

List of Back Coating Layer Forming Composition Non-magnetic inorganicpowder (α-iron oxide) 85.0 parts Surface treatment layer: Al₂O₃, SiO₂Average particle size: 0.15 μm Tap density: 0.8 Average acicular ratio:7 BET specific surface area: 52 m²/g pH: 8 DBP oil absorption: 33 g/100g Fine particle carbon black 20.0 parts (average particle size: 16 nm)Coarse particle carbon black none (average particle size: 370 nm) Vinylchloride copolymer (MR 104 manufactured 13.0 parts by KanekaCorporation) Polyurethane resin (VYLON UR820 manufactured 6.0 parts byToyobo Co., Ltd.) Phenylphosphonic acid 3.0 parts Alumina powder(average particle size: 0.25 μm) 5.0 parts Cyclohexanone 140.0 partsMethyl ethyl ketone 170.0 parts Butyl stearate 2.0 parts Stearic acid1.0 part

Preparation of Each Layer Forming Composition

For each of the magnetic layer forming composition, the non-magneticlayer forming composition, and the back coating layer formingcomposition, each component was kneaded for 240 minutes by an openkneader, and then dispersed with a sand mill. The dispersion time was720 minutes for the magnetic layer forming composition, and was 1080minutes for each of the non-magnetic layer forming composition and theback coating layer forming composition. 4.0 parts of trifunctional lowmolecular weight polyisocyanate (CORONATE 3041 manufactured by TosohCorporation) was added to the dispersion liquid obtained as describedabove, the mixture was further stirred and mixed for 20 minutes, andthen was filtered using a filter having a pore diameter of 0.5 μm.

Thus, a magnetic layer forming composition, a non-magnetic layer formingcomposition, and a back coating layer forming composition were prepared.

Manufacturing of Magnetic Tape Cartridge

The non-magnetic layer forming composition was applied onto a surface ofa biaxially stretched polyethylene naphthalate support having athickness of 4.6 μm (type: see Table 2) and dried under an environmentat an atmosphere temperature of 100° C. so that a thickness after dryingbecomes 0.7 μm, and thus a non-magnetic layer was formed. The magneticlayer forming composition was applied onto a surface of the non-magneticlayer so that a thickness after drying becomes 60 nm, and thus a coatinglayer of the magnetic layer forming composition was formed. While thiscoating layer is in a wet (undried) state, a vertical orientationprocess was performed in which a magnetic field of a magnetic fieldintensity of 0.6 T was applied in a direction perpendicular to a surfaceof the coating layer. Thereafter, the coating layer was dried to form amagnetic layer. The back coating layer forming composition was appliedonto the surface of the support on a side opposite to the surfaceprovided with the non-magnetic layer and the magnetic layer and driedunder an environment at an atmosphere temperature of 120° C. (dryingtemperature) so that a thickness after the drying becomes 0.4 μm, andthus a back coating layer was formed.

After that, a calendering process was performed using a seven-stagecalender machine configured with only a metal roll, under conditions ofa calendering speed of 100 m/min, a linear pressure of 350 kg/cm (1kg/cm is 0.98 kN/m), and a calender temperature of 100° C. (a surfacetemperature of a calendering roll). Then, after a thermal process wasperformed in an environment of an atmosphere temperature of 70° C. for24 hours, slitting was performed so as to have a width of ½ inches (1inch is 0.0254 meters). In a state where the magnetic layer of themagnetic tape obtained by slitting was demagnetized, a servo patternhaving a disposition and a shape according to an LTO Ultrium format wasformed on the magnetic layer by a servo write head mounted on acommercially available servo writer. Accordingly, a magnetic tapeincluding data bands, servo bands, and guide bands in the dispositionaccording to the LTO Ultrium format in the magnetic layer, and includingservo patterns (timing-based servo pattern) having the disposition andthe shape according to the LTO Ultrium format on the servo band wasobtained. The obtained magnetic tape was wound on a reel of a magnetictape cartridge (LTO Ultrium 7 data cartridge) to produce a single reeltype magnetic tape cartridge of Example 1 in which a magnetic tapehaving a length of 950 m was wound around the reel.

Examples 2 to 7 and Comparative Examples 1 to 4

A magnetic tape cartridge was manufactured in the same manner as inExample 1 except that the type of the ferromagnetic powder of themagnetic layer and/or the type of the non-magnetic support was changedas shown in Table 2.

Examples 8 to 11

A magnetic tape cartridge was manufactured in the same manner as inExample 1 except that the content and/or the type of carbon black of theback coating layer was changed as shown in Table 3.

Examples 12 to 16

A magnetic tape cartridge was manufactured in the same manner as inExample 1, except that the drying time and/or the calenderingtemperature after application of the back coating layer formingcomposition was changed as shown in Table 4.

Examples 17 to 19

A magnetic tape cartridge was manufactured in the same manner as inExample 1 except that the content of carbon black of the magnetic layerforming composition and/or the dispersion time of the back coating layerforming composition was changed as shown in Table 5.

Example 20

A magnetic tape cartridge was manufactured in the same manner as inExample 1 except that the ferromagnetic powder of the magnetic layerforming composition was changed to the hexagonal barium ferrite powdershown in Table 5.

Evaluation Method

(1) Various PSDs on Surface of Magnetic Layer and Surface of BackCoating Layer

Measurement was performed under the conditions shown in Table 1 using anon-contact optical roughness measuring instrument, and aPSD_(5μm-PSDmag), a PSD_(10μm-PSDbc), a PSD_(3μm-PSDbc), and aPSD_(10μm-PSDbc) described in the table which will be described laterwere obtained. The PSD of the support shown in the table which will bedescribed later is a value obtained in the same manner for a measurementsample obtained from a support original material cut out from thesupport used for manufacturing the magnetic tape. Regarding thecalculation of PSD at a specific pitch, for example, in obtaining a PSDof 10 μm, in a case where the obtained measurement results have, forexample, only points of 9.9 μm and 10.3 μm, an arithmetic average of themeasurement values at the point of 10 μm by rounding off the firstdecimal place is to be adopted.

TABLE 1 Non-contact Conter GT-I manufactured by Bruker Japan K.K.optical roughness measuring instrument Objective lens ×50 Intermediatelens ×0.55 Measurement 167 μm × 125 μm area Distortion Cylinder and Tiltcorrection Sampling length 130 nm Data analysis Vision64 softwareCorrection Measurement is performed after accuracy confirmation usingbelow standard. Step Height Standard (Model SHS-1.8 QC) manufactured byVLSI Standards, Inc. SiC Reference Flat manufactured by Zygo CorporationMeasurement Tape longitudinal direction is measured direction length of167 μm. PSD calculation PSD in tape longitudinal direction iscalculated. direction Sample sticking Magnetic tape is placed on glassplate, and method four corners of magnetic tape are stuck by adhesivetape so that no slack is visually checked. Measurement Width directionposition: position and Central portion in tape width direction number ofLongitudinal direction position: measurements Magnetic tape is takenfrom magnetic tape cartridge, and measurement is performed 5 times ateach of three positions of 50 m, 450 m, and 900 m from outermost outerend opposite to inner end wound around reel of magnetic tape cartridge.From measurement results obtained at three measurement positions, PSD isarithmetic average of measurement values (therefore, three measurementvalues for one measurement position, nine measurement values in totalfor three measurement positions) excluding minimum value and maximumvalue among PSDs obtained by five measurements at each measurementposition.

(2) Kurtosis and Skewness of Surface of Magnetic Layer and Surface ofBack Coating Layer

A kurtosis Rku_(mag), a kurtosis Rku_(bc), a skewness Rsk_(mag), and askewness Rsk_(bc) shown in the table which will be described later arevalues obtained according to JIS B 0601:2013 from the measurementresults obtained by measurement under the conditions shown in Table 1above. The number of measurements was the same as described in Table 1,and an arithmetic average of a total of nine measurement values wasadopted.

(3) Evaluation of Recording and Reproducing Quality During RepeatedRunning

Each magnetic tape cartridge of the examples and the comparativeexamples shown in Table 2 was mounted on an LTO Ultrium 7 (LTO 7) drive,the entire length of the magnetic tape was reciprocated 10,000 timeswith the LTO 7 drive, and then it is confirmed whether specifiedcapacity recording could be possible. A specified capacity is 6.0 TB(terabytes). A case where the specified capacity recording could bepossible without an error during recording was evaluated as “OK”, and acase where the specified capacity recording could not be possiblebecause the drive stopped due to an error signal was evaluated as “NG”.

Further, a central portion in a longitudinal direction of the tape (400to 500 m from the outer end of the tape) after the above evaluation withLTO 7 drive was cut out, and the magnetic layer surface was observedwith a differential interference microscope (observation region: 2.0mm×1.5 mm). A case where no damage is observed on the surface wasevaluated as “AA”, a case of 1 or 2 damages was evaluated as “A”, a caseof 3 or 4 damages was evaluated as “B”, a case of 5 to 9 damages wasevaluated as “C”, and a case of 10 or more damages was evaluated as “D”.

Evaluation results shown in Tables 3 to 5 are evaluation results afterincreasing the number of reciprocating runs to 20,000 times.

TABLE 2 Example Example Example Example Example Example 1 2 3 4 5 6Ferro- Type SrFe1 SrFe1 SrFe1 SrFe1 SrFe1 SrFe2 magnetic Averageparticle 900    900    900    900    900    1100    powder volume (nm³)Non- Type a b c d e a magnetic Magnetic surface side 1.82E+03 3.10E+021.89E+03 1.77E+03 1.02E+04 1.82E+03 support 5 μm PSD (nm³) Back surfaceside 3.64E+04 3.56E+04 1.58E+06 7.50E+02 3.69E+04 3.64E+04 10 μm PSD(nm³) Magnetic Hk (kOe) 25   25   25   25   25   19   layer PSD Ratio(PSD_(5 μm-PSDmag)/  0.033   0.0052   0.0053  0.19  0.20  0.038PSD_(10 μm-PSDbc)) Magnetic layer 2.48E+03 3.96E+02 2.21E+03 2.31E+031.42E+04 2.62E+03 PSD_(5 μm-PSDmag) Back coating layer 7.52E+04 7.62E+044.17E+05 1.22E+04 7.10E+04 6.89E+04 PSD_(10 μm-PSDbc) Evaluation Numberof damages AA C B B C A Recording and OK OK OK OK OK OK reproducingquality during repeated running Example Comparative ComparativeComparative Comparative 7 Example 1 Example 2 Example 3 Example 4 Ferro-Type ϵ-Iron SrFe1 SrFe1 SrFe1 SrFe1 magnetic oxide powder Averageparticle 750    900    900    900    900    volume (nm³) Non- Type a g fh i magnetic Magnetic surface side 1.82E+03 1.43E+02 1.80E+03 1.75E+031.48E+04 support 5 μm PSD (nm³) Back surface side 3.64E+04 3.61E+044.07E+06 3.20E+02 3.56E+04 10 μm PSD (nm³) Magnetic Hk (kOe) 30   25  25   25   25   layer PSD Ratio (PSD_(5 μm-PSDmag)/  0.036   0.0038  0.0042  0.23  0.22 PSD_(10 μm-PSDbc)) Magnetic layer 2.50E+03 2.69E+022.49E+03 2.00E+03 1.67E+04 PSD_(5 μm-PSDmag) Back coating layer 6.94E+047.07E+04 5.93E+05 8.70E+03 7.59E+04 PSD_(10 μm-PSDbc) Evaluation Numberof damages A D D D D Recording and OK NG NG NG NG reproducing qualityduring repeated running

TABLE 3 Example 1 Example 8 Example 9 Example 10 Example 11Ferromagnetic Type SrFe1 SrFe1 SrFe1 SrFe1 SrFe1 powder Average particlevolume (nm³) 900 900 900 900 900 Content of Fine particle carbon black20.0 12.0 8.0 20.0 20.0 carbon black (average particle size of 16 nm) ofback coating Coarse particle carbon black 0 0 0 2.0 4.0 layer (part)(average particle size of 370 nm) Non-magnetic Type a a a a a supportMagnetic surface side 5 μm PSD (nm³) 1.82E+03 1.82E+03 1.82E+03 1.82E+031.82E+03 Back surface side 10 μm PSD (nm³) 3.64E+04 3.64E+04 3.64E+043.64E+04 3.64E+04 Magnetic Hk (kOe) 25 25 25 25 25 layer PSD Ratio(PSD_(5 μm-PSDmag)/PSD_(10 μm-PSDbc)) 0.033 0.029 0.029 0.043 0.043Ratio (PSD_(3 μm-PSDmag)/PSD_(10 μm-PSDbc)) 0.46 0.053 0.046 0.72 0.78Magnetic layer PSD_(5 μm-PSDmag) 2.48E+03 2.20E+03 2.19E+03 2.42E+032.47E+03 Back coating layer PSD_(10 μm-PSDbc) 7.52E+04 7.59E+04 7.55E+045.63E+04 5.74E+04 Back coating layer PSD_(3 μm-PSDbc) 3.46E+04 4.02E+033.47E+03 4.05E+04 4.48E+04 Evaluation Number of damages AA B C B CRecording and reproducing quality OK OK OK OK OK during repeated running

TABLE 4 Example 1 Example 12 Example 13 Example 14 Example 15 Example 16Ferromagnetic Type SrFe1 SrFe1 SrFe1 SrFe1 SrFe1 SrFe1 powder Averageparticle volume (nm³) 900 900 900 900 900 900 Content of Fine particlecarbon black 20.0 20.0 20.0 20.0 20.0 20.0 carbon black of (averageparticle size of 16 nm) back coating Coarse particle carbon black 0 0 00 0 0 layer (part) (average particle size of 370 nm) Non-magnetic Type aa a a a a support Magnetic surface side 1.82E+03 1.82E+03 1.82E+031.82E+03 1.82E+03 1.82E+03 5 μm PSD (nm³) Back surface side 3.64E+043.64E+04 3.64E+04 3.64E+04 3.64E+04 3.64E+04 10 μm PSD (nm³) Calenderingtemperature (° C.) 100 110 120 80 70 100 Drying temperature of backcoating layer 120 120 120 120 120 90 forming composition (° C.) Magneticlayer Hk(kOe) 25 25 25 25 25 25 PSD Ratio (PSD_(5 μm-PSDmag)/ 0.0330.028 0.027 0.031 0.031 0.033 PSD_(10 μm-PSDbc)) Ratio(PSD_(3 μm-PSDmag)/ 0.46 0.42 0.41 0.45 0.45 0.46 PSD_(10 μm-PSDbc))Magnetic layer PSD_(5 μm-PSDmag) 2.48E+03 1.98E+03 1.88E+03 2.98E+032.98E+03 2.48E+03 Back coating layer PSD_(10 μm-PSDbc) 7.52E+04 7.12E+047.02E+04 9.61E+04 9.61E+04 7.52E+04 Back coating layer PSD_(3 μm-PSDbc)3.46E+04 2.99E+04 2.88E+04 4.33E+04 4.33E+04 3.46E+04 Kurtosis Product(Rku_(mag) × Rku_(bc)) 10.7 7.2 6.8 19.6 20.3 9.0 Magnetic layerRku_(mag) 3.03 2.63 2.53 3.94 3.94 3.10 Back coating layer Rku_(bc) 3.532.72 2.67 4.97 5.14 2.91 Evaluation Number of damages AA A B A B BRecording and reproducing OK OK OK OK OK OK quality during repeatedrunning

TABLE 5 Example 1 Example 17 Example 18 Example 19 Example 20Ferromagnetic powder Type SrFe1 SrFe1 SrFe1 SrFe1 SrFe1 Average particlevolume (nm³) 900 900 900 900 1100 Content of carbon black of magneticlayer (part) 1.0 0.2 1.0 0.2 1.0 Content of carbon black of Fineparticle carbon black 20 20 20 20 20 back coating layer (part) (averageparticle size of 16 nm) Coarse particle carbon black 0 0 0 0 0 (averageparticle size of 370 nm) Dispersion time of back coating layer formingcomposition (minute) 1080 1080 2160 2160 1080 Non-magnetic support Typea a a a a Magnetic surface side 5 μm PSD (nm³) 1.82E+03 1.82E+031.82E+03 1.82E+03 1.82E+03 Back surface side 10 μm PSD (nm³) 3.64E+043.64E+04 3.64E+04 3.64E+04 3.64E+04 Calendering temperature (° C.) 100100 100 100 100 Drying temperature of back coating layer formingcomposition (° C.) 120 120 120 120 120 Magnetic layer Hk (kOe) 25 25 2525 13 PSD Ratio (PSD_(5 μm-PSDmag)/PSD_(10 μm-PSDbc)) 0.033 0.029 0.0350.031 0.032 Ratio (PSD_(3 μm-PSDmag)/PSD_(10 μm-PSDbc)) 0.46 0.41 0.390.38 0.47 Magnetic layer PSD_(5 μm-PSDmag) 2.48E+03 2.28E+03 2.38E+032.17E+03 2.25E+03 Back coating layer PSD_(10 μm-PSDbc) 7.52E+04 7.86E+046.80E+04 7.00E+04 7.03E+04 Back coating layer PSD_(3 μm-PSDbc) 3.46E+043.22E+04 2.65E+04 2.66E+04 3.30E+04 Kurtosis Product (Rku_(mag) ×Rku_(bc)) 10.7 10.0 10.7 10.0 11.0 Magnetic layer Rku_(mag) 3.03 3.013.03 3.01 3.21 Back coating layer Rku_(bc) 3.53 3.33 3.53 3.33 3.44Skewness Magnetic layer Rsk_(mag) 0.18 −0.22 0.23 −0.24 0.21 Backcoating layer Rsk_(bc) 0.48 0.44 −0.33 −0.35 0.44 Evaluation Number ofdamages AA AA B B AA Recording and reproducing quality OK OK OK OK OKduring repeated running

From the results shown in Tables 2 to 5, it can be confirmed thatExamples 1 to 20 are excellent in recording and reproducing qualityduring repeated running as compared with Comparative Examples 1 to 4.

The same evaluation was performed in a case where ferromagnetic powdersof Comparative Examples 1 to 4 were changed to the same ferromagneticpowder as ferromagnetic powder (ε-iron oxide powder) used in Example 7and the like, and as a result, the specified capacity recording couldnot be possible because the drive stopped due to an error signal(evaluation result NG).

With respect to this, the same evaluation was performed in a case whereferromagnetic powders of Comparative Examples 1 to 4 were changed to thesame ferromagnetic powder as ferromagnetic powder (hexagonal bariumferrite powder) used in Example 20, and as a result, the specifiedcapacity recording could be possible, but an error occurred duringrecording.

It is considered that the above results show that deterioration ofrecording and reproducing quality during repeated running tends to bemanifested in a case where ferromagnetic powder selected from the groupconsisting of hexagonal strontium ferrite powder and ε-iron oxide powderwas used as ferromagnetic powder of a magnetic layer. With respect tothis, such deterioration of recording and reproducing quality duringrepeated running could be suppressed in Examples 1 to 19 as shown in theabove tables.

An aspect of the present invention is effective in a technical field ofa magnetic tape for high-density recording.

What is claimed is:
 1. A magnetic tape comprising: a non-magneticsupport; a magnetic layer that includes ferromagnetic powder above onesurface side of the non-magnetic support; a non-magnetic layer thatincludes non-magnetic powder between the magnetic layer and thenon-magnetic support; and a back coating layer that includesnon-magnetic powder on the other surface side of the non-magneticsupport, wherein the ferromagnetic powder is ferromagnetic powderselected from the group consisting of hexagonal ferrite powder andε-iron oxide powder, a thickness of the non-magnetic layer is 0.1 to 1.0μm, a ratio PSD_(5μm-PSDmag)/PSD_(10μm-PSDbc) of a PSD_(5μm-PSDmag) at a5 μm pitch on a surface of the magnetic layer and a PSD_(10μm-PSDbc) ata 10 μm pitch on a surface of the back coating layer is in a range of0.0052 to 0.20, and the PSD or power spectrum density at a given pitchin microns is obtained by: measuring profile data of a measurementtarget surface using a non-contact optical interference type surfaceroughness machine, the target surface being either the magnetic layer orthe back coating layer; calculating a PSD in a longitudinal direction ofthe magnetic tape; employing a mounting function of the machine to carryout a Fourier transformation of the profile data in the longitudinaldirection, and calculate a PSD; and from this PSD, calculating a PSDvalue at each wavelength to obtain a PSD value corresponding to thegiven pitch in microns.
 2. The magnetic tape according to claim 1,wherein a ratio PSD_(3μm-PSDbc)/PSD_(10μm-PSDbc) of a PSD_(3μm-PSDbc) ata 3 μm pitch on a surface of the back coating layer and aPSD_(10μm-PSDbc) at a 10 μm pitch on a surface of the back coating layeris in a range of 0.050 to 0.75.
 3. The magnetic tape according to claim1, wherein a product Rku_(mag)×Rku_(bc) of a kurtosis Rku_(mag) of asurface of the magnetic layer and a kurtosis Rku_(bc) of a surface ofthe back coating layer is in a range of 7.0 to 20.0, and the kurtosisRku is a value obtained according to JIS B 0601:2013 from profile dataof a surface roughness in a longitudinal direction of the magnetic tapeobtained for a region having an area 167 μm×125 μm on a surface of ameasurement target layer by using a non-contact optical interferencetype surface roughness machine.
 4. The magnetic tape according to claim3, wherein the kurtosis Rku_(mag) of the surface of the magnetic layerand the kurtosis Rku_(bc) of the surface of the back coating layer havea relationship of Rku_(mag)<Rku_(bc).
 5. The magnetic tape according toclaim 1, wherein at least one of a skewness Rsk_(mag) of a surface ofthe magnetic layer or a skewness Rsk_(bc) of a surface of the backcoating layer is 0 or more.
 6. The magnetic tape according to claim 5,wherein the skewness Rsk_(bc) of the surface of the back coating layeris 0 or more.
 7. The magnetic tape according to claim 1, wherein thehexagonal ferrite powder is hexagonal strontium ferrite powder.
 8. Themagnetic tape according to claim 1, wherein the thickness of thenon-magnetic layer is 0.1 to 0.7 μm.
 9. A magnetic tape cartridgecomprising: the magnetic tape according to claim
 1. 10. The magnetictape cartridge according to claim 9, wherein a ratioPSD_(3μm-PSDbc)/PSD_(10μm-PSDbc) of a PSD_(3μm-PSDbc) at a 3 μm pitch ona surface of the back coating layer and a PSD_(10μm-PSDbc) at a 10 μmpitch on a surface of the back coating layer is in a range of 0.050 to0.75.
 11. The magnetic tape cartridge according to claim 9, wherein aproduct Rku_(mag)×Rku_(bc) of a kurtosis Rku_(mag) of a surface of themagnetic layer and a kurtosis Rku_(bc) of a surface of the back coatinglayer is in a range of 7.0 to 20.0, and the kurtosis Rku is a valueobtained according to JIS B 0601:2013 from profile data of a surfaceroughness in a longitudinal direction of the magnetic tape obtained fora region having an area 167 μm×125 μm on a surface of a measurementtarget layer by using a non-contact optical interference type surfaceroughness machine.
 12. The magnetic tape cartridge according to claim11, wherein the kurtosis Rku_(mag) of the surface of the magnetic layerand the kurtosis Rku_(bc) of the surface of the back coating layer havea relationship of Rku_(mag)<Rku_(bc).
 13. The magnetic tape cartridgeaccording to claim 9, wherein at least one of a skewness Rsk_(mag) of asurface of the magnetic layer or a skewness Rsk_(bc) of a surface of theback coating layer is 0 or more, and the skewness Rsk is a valueobtained according to JIS B 0601:2013 from profile data of a surfaceroughness in a longitudinal direction of the magnetic tape obtained fora region having an area 167 μm×125 μm on a surface of a measurementtarget layer by using a non-contact optical interference type surfaceroughness machine.
 14. The magnetic tape cartridge according to claim13, wherein the skewness Rsk_(bc) of the surface of the back coatinglayer is 0 or more.
 15. The magnetic tape cartridge according to claim9, wherein the hexagonal ferrite powder is hexagonal strontium ferritepowder.
 16. The magnetic tape cartridge according to claim 9, whereinthe thickness of the non-magnetic layer is 0.1 to 0.7 μm.